Methods and systems for thermal-based laser processing a multi-material device

ABSTRACT

A method and system for locally processing a predetermined microstructure formed on a substrate without causing undesirable changes in electrical or physical characteristics of the substrate or other structures formed on the substrate are provided. The method includes providing information based on a model of laser pulse interactions with the predetermined microstructure, the substrate and the other structures. At least one characteristic of at least one pulse is determined based on the information. A pulsed laser beam is generated including the at least one pulse. The method further includes irradiating the at least one pulse having the at least one determined characteristic into a spot on the predetermined microstructure. The at least one determined characteristic and other characteristics of the at least one pulse are sufficient to locally process the predetermined microstructure without causing the undesirable changes.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of U.S. provisionalapplication Serial No. 60/279,644, filed Mar. 29, 2001, entitled “Methodand System for Severing Highly Conductive Micro-Structures.” Thisapplication is related to U.S. patent application Ser. No. ______, filedon the same day as this application, entitled “Method and System forProcessing One or More Microstructures of a Multi-Material Device.”

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to the field of laser processingmethods and systems, and specifically, to laser processing methods andsystems for thermal-based laser processing multi-material devices.

[0004] 2. Background Art

[0005] In the repair of memory integrated circuits such as DRAMs andlaser programming of high-density logic devices, the use of newmaterials, such as aluminum, gold, and copper, coupled with the smallgeometry of these devices, make the problem of link removal difficult.The new materials are typically metals or highly conductive compositeshaving reflectivity that is well over 90% in the visible and nearinfrared wavelength regions. Aluminum, for example, reflects greaterthan 90% of the laser energy over the range from the UV through to thenear infrared. Gold and copper reflects even more strongly in the nearinfrared, the wavelengths of choice used by most of the lasers repairingmemories in production.

[0006] Further, economics and device performance have driven the sizefor the DRAMs and logic devices to very small physical dimensions. Notonly are the devices small, but the interconnects and links thicknesshave also decreased dramatically in recent years.

[0007] Thermal laser processing of links relies on the differentialthermal expansion between the oxide above the link and the link itself.This differential expansion results in a high pressure build-up of themolten link contained by the oxide. The oxide over the link is necessaryto contain the link in a molten state long enough to build-up sufficientpressure to crack the oxide and explosively expel the link material. Ifthe pressure is too low, the link will not be removed cleanly.Alternative laser wavelengths and laser control strive to increase thelaser “energy window” without damaging the substrate and materialcontiguous to the link.

[0008] Descriptions of an all-copper, dual-Damascene process technologycan be found in “Benefits of Copper—Copper Technology is Here Today inWorking Devices,” NOVELLUS DAMASEUS, Dec. 20, 2001; and “PreventingCross-Contamination Caused By Copper Diffusion and Other Sources,” P.Cacouvis, MICRO, July 1999.

[0009]FIGS. 2a and 2 b illustrate prior art laser processing ofmulti-layer structure wherein a target structure is located in proximityto a substrate, with a q-switched pulse 20 from a conventional solidstate laser 21 irradiating and overfilling a target structure 23. Alaser spot size is typically significantly larger than the (target) linksize which relaxes precision positioning requirements. A laserwavelength is typically selected based on substrate 27 (commonlySilicon) transmission so as to allow for higher peak laser power orother system and process variations. In certain cases, a layer 28,25absorption coefficient is controlled (e.g., as a transition orprotective layer) and/or a wavelength selected wherein substrate damageis avoided.

[0010] Further information is available regarding link blowing methodsand systems, including material processing, system design, and devicedesign considerations, in the following representative U.S. patents andpublished U.S. applications: U.S. Pat. Nos. 4,399,345; 4,532,402;4,826,785; 4,935,801; 5,059,764; 5,208,437; 5,265,114; 5,473,624;6,057,180; 6,172,325; 6,191,486; 6,239,406; 2002-0003130; and2002-0005396.

[0011] Other representative publications providing background on linkprocessing of memory circuits or similar laser processing applicationsinclude: “Laser Adjustment of Linear Monolithic Circuits,” Litwin andSmart, ICAELO, (1983); “Computer Simulation of Target Link Explosion InLaser Programmable Memory,” Scarfone, Chlipala (1986); “Precision LaserMicromachining,” Boogard, SPIE Vol. 611 (1986); “Laser Processing forApplication Specific Integrated Circuits (asics),” SPIE Vol. 774, Smart(1987); “Xenon Laser Repairs Liquid Crystal Displays,” Waters, Laser andOptronics, (1988); “Laser Beam Processing and Wafer Scale Integration,”Cohen (1988); “Optimization of Memory Redundancy Link Processing,” Sun,Harris, Swenson, Hutchens, Vol. SPIE 2636, (1995); “Analysis of LaserMetal Cut Energy Process Window,” Bernstein, Lee, Yang, Dahmas, IEEETrans. On Semicond. Manufact., Vol 13, No. 2. (2000).

[0012] Also, the following co-pending U.S. applications and issuedpatents are assigned to the assignee of the present invention and arehereby incorporated by reference in their entirety:

[0013] 1. U.S. Pat. No. 5,300,756, entitled “Method and System forSevering Integrated-Circuit Connection Paths by a Phase Plate AdjustedLaser beam”;

[0014] 2. U.S. Pat. No. 6,144,118, entitled “High Speed PrecisionPositioning Apparatus”;

[0015] 3. U.S. Pat. No. 6,181,728, entitled “Controlling LaserPolarization”;

[0016] 4. U.S. Pat. No. 5,998,759, entitled “Laser Processing”;

[0017] 5. U.S. Pat. No. 6,281,471, entitled “Energy Efficient,Laser-Based Method and System for Processing Target Material”;

[0018] 6. U.S. Pat. No. 6,340,806, entitled “Energy-Efficient Method andSystem for Processing Target Material Using an Amplified,Wavelength-Shifted Pulse Train”;

[0019] 7. U.S. Ser. No. 09/572,925, entitled “Method and System ForPrecisely Positioning A Waist of A Material-Processing Laser Beam ToProcess Microstructures Within A Laser-Processing Site”, filed May 16,2000, and published as WO 0187534 A2, December, 2001;

[0020] 8. U.S. Pat. No. 6,300,590, entitled “Laser Processing”; and

[0021] 9. U.S. Pat. No. 6,339,604, entitled “Pulse Control in LaserSystems.”

[0022] However, it is to be understood that this listing is not anadmission that any of the above references are prior art under thePatent Statute.

[0023] The subject matter of the above referenced applications andpatents is related to the present invention. References to the abovepatents and applications are cited by reference number in the followingsections.

SUMMARY OF THE INVENTION

[0024] An object of the present invention is to provide improved methodsand systems for thermal-based laser processing multi-material devices.

[0025] In carrying out the above object and other objects of the presentinvention, a method for thermal-based laser processing a multi-materialdevice including a substrate and at least one microstructure isprovided. The processing occurs with multiple pulses in a single passoperation controlled with a positioning subsystem of a thermalprocessing system. The positioning subsystem induces relative motionbetween the device and laser beam waists. The processing removes the atleast one microstructure without damaging the substrate. The methodincludes generating a first pulse having a first predeterminedcharacteristic, and irradiating the at least one microstructure with thefirst pulse wherein a first beam waist associated with the first pulseand the at least one microstructure substantially coincide. The step ofirradiating at least initiating processing of the at least onemicrostructure. The method also includes generating a second pulsehaving a second predetermined characteristic. The second pulse isdelayed a predetermined time relative to the first pulse. The methodfurther includes irradiating the at least one microstructure with thesecond pulse wherein a second beam waist associated with the secondpulse and the at least one microstructure substantially coincide. Thestep of irradiating the at least one microstructure with the secondpulse further processing the at least one microstructure wherein theprocessing of the at least one microstructure with the first and secondpulses occurs during relative motion of the at least one microstructureand the beam waists in a single pass whereby throughput of the thermalprocessing system is substantially improved.

[0026] The device may be a semiconductor memory including a siliconsubstrate and the at least one microstructure may be a metal link of thesemiconductor memory separated from the silicon substrate by at leastone oxide layer.

[0027] At least one of the pulses may have a duration of greater than afew picoseconds to several nanoseconds.

[0028] The pulses may be generated by a mode-locked laser system andamplified with an optical amplifier.

[0029] At least one of the pulses may be generated by a q-switchedmicrolaser having a pulsewidth less than 5 nanoseconds.

[0030] The first and second pulses may be propagated along differentoptical paths so that the second pulse is delayed for the predeterminedtime relative to the first pulse based on a difference in optical pathlength.

[0031] The pulses may have a temporal spacing less than or approximatelyequal to the predetermined time. The method further include selectingthe second pulse to irradiate the at least one microstructure.

[0032] The predetermined time may be determined by a thermal property ofthe substrate wherein substrate temperature is substantially reducedafter the predetermined time compared to the temperature of thesubstrate during the step of irradiating the at least one microstructurewith the second pulse.

[0033] The substrate temperature may be substantially reduced toapproximately room temperature.

[0034] The first and second predetermined characteristics may include asubstantially square temporal pulse shape having a rise time of lessthan about 2 nanoseconds and a pulse duration of about 10 nanoseconds.

[0035] The predetermined time may be in the range of about 20-50nanoseconds, or may be in the range of about 30 nanoseconds.

[0036] Two pulses may be used to completely process the at least onemicrostructure, and laser energy of each of the pulses is about 60-70%of laser energy required for laser processing the at least onemicrostructure with a single pulse.

[0037] Relative position change between the pulses at the at least onemicrostructure may be less than about 10% of a dimension of the at leastone microstructure to be processed.

[0038] At least one of the first and second predeterminedcharacteristics may include a substantially square pulse.

[0039] At least one of the predetermined characteristics may include anoncircular spatial profile based on a selected numerical aperture andshape of a spot and the spot and the at least one microstructure aresubstantially correlated in at least one dimension whereby percent oflaser energy delivered to the at least one microstructure is increasedand irradiance of the substrate is decreased.

[0040] A spatial beam shape of the second pulse may be in the form of acleaning beam having an energy density lower than energy density of thefirst pulse.

[0041] The cleaning beam may have an attenuated central region and ahigher energy outer region so as to remove debris surrounding a targetsite on the at least one microstructure.

[0042] The steps of generating may include directing a portion of alaser pulse through an optical subsystem having opposing, spaced-apart,corner cube reflectors and polarization rotators so as to align a pulsedlaser beam, and to control delay and amplitude of the second pulserelative to the first pulse.

[0043] The steps of generating may further include providing an opticalsubsystem having multiple lasers wherein delay between trigger pulses tothe optical subsystem determines the predetermined time.

[0044] A fiber optic delay line may delay the second pulse for thepredetermined time and the predetermined time may be about severalnanoseconds.

[0045] Relative position change between the pulses at the at least onemicrostructure may be either greater than about 10% of a dimension ofthe at least one microstructure to be processed or greater than about ½of either of the beam waists and may further include a high speed beamdeflector operatively coupled to the positioning subsystem to compensatefor relative motion between the pulses. The second pulse may bedeflected by the deflector to also substantially irradiate the at leastone microstructure with the second pulse.

[0046] The predetermined time may be in the range of about 10 ns to 10μs.

[0047] The beam deflector may be a single axis acousto-optic device.

[0048] The first and second predetermined characteristics may be basedon physical properties of the multi-material device.

[0049] The first pulse may irradiate a first portion of the at least onemicrostructure and the second pulse may irradiate a second portion ofthe at least one microstructure, and relative position change betweenthe first and second portions of the at least one microstructure may beless than ¼ of either of the beam waists.

[0050] The step of providing may also provide at least one opticalamplifier optically coupled to at least one of the lasers.

[0051] The at least one microstructure and the beam waists may berelatively positioned during relative motion based uponthree-dimensional information.

[0052] The steps of generating may include generating a single pulse andforming the first and second pulses from the single pulse.

[0053] The step of forming may delay the second pulse for thepredetermined time relative to the first pulse.

[0054] The step of forming may include splitting the single pulse with amulti-frequency deflector to form the first and second pulses.

[0055] First and second microstructures may be irradiated by the firstand second pulses, respectively.

[0056] Further in carrying out the above object and other objects of thepresent invention, a system for thermal-based laser processing amulti-material device including a substrate and at least onemicrostructure is provided. The processing occurs with multiple pulsesin a single pass operation controlled with a positioning subsystem whichinduces relative motion between the device and laser beam waists. Theprocessing removes the at least one microstructure without damaging thesubstrate. The system includes means for generating a first pulse havinga first predetermined characteristic, and means for irradiating the atleast one microstructure with the first pulse wherein a first beam waistassociated with the first pulse and the at least one microstructuresubstantially coincide. The first pulse at least initiating processingof the at least one microstructure. The system also includes means forgenerating a second pulse having a second predetermined characteristic.The second pulse is delayed a predetermined time relative to the firstpulse. The system further includes means for irradiating the at leastone microstructure with the second pulse wherein a second beam waistassociated with the second pulse and the at least one microstructuresubstantially coincide. The second pulse further processing the at leastone microstructure wherein the processing of the at least onemicrostructure with the first and second pulses occurs during relativemotion of the at least one microstructure and the beam waists in asingle pass whereby throughput of the system is substantially improved.

[0057] The means for generating may include a mode-locked laser systemand may further include an optical amplifier for amplifying the pulses.

[0058] At least one of the means for generating may include a q-switchedmicrolaser having a pulsewidth less than 5 nanoseconds.

[0059] The pulses may have a temporal spacing less than or approximatelyequal to the predetermined time. The system may further include meansfor selecting the second pulse to irradiate the at least onemicrostructure.

[0060] The predetermined time may be determined by a thermal property ofthe substrate wherein substrate temperature may be substantially reducedafter the predetermined time compared to the temperature of thesubstrate during irradiation of the at least one microstructure with thesecond pulse.

[0061] The means for generating the first and second pulses may includean optical subsystem having opposing, spaced-apart, corner cubereflectors and polarization rotators so as to align a pulsed laser beam,and to control delay and amplitude of the second pulse relative to thefirst pulse.

[0062] The means for generating the first and second pulses may alsoinclude an optical subsystem having multiple lasers wherein delaybetween trigger pulses to the optical subsystem determines thepredetermined time.

[0063] The means for generating the first and second pulses may furtherinclude means for generating a single pulse and means for forming thefirst and second pulses from the single pulse.

[0064] The means for forming may include a multi-frequency deflector forsplitting the single pulse to form the first and second pulses.

[0065] Still further in carrying out the above object and other objectsof the present invention, a method for thermal-based laser processing amulti-material device including a substrate and a microstructure isprovided. The method includes generating the at least one laser pulsehaving at least one predetermined characteristic based on a differentialthermal property of materials of the device. The method also includesirradiating the microstructure with the at least one laser pulse whereina first portion of the at least one pulse increases a difference intemperature between the substrate and the microstructure, and a secondportion of the at least one pulse further increases the difference intemperature between the substrate and the microstructure to process themulti-material device without damaging the substrate.

[0066] The first and second portions may be portions of a single pulse,or may be portions of different pulses.

[0067] The first portion of the at least one pulse may increasetemperature of the microstructure.

[0068] The first portion may be a high density leading edge portion ofthe at least one pulse.

[0069] The leading edge portion may have a rise time of less than twonanoseconds.

[0070] The rise time may be less than one nanosecond.

[0071] The first and second portions of the at least one pulse may besufficient to remove the microstructure.

[0072] The microstructure may be a metal link having reflectivity, andthe leading edge portion of the at least one pulse may reduce thereflectivity of the metal link.

[0073] The substrate may be silicon and the device may be asemiconductor memory.

[0074] The second portion of the at least one pulse may further increasethe temperature of the microstructure.

[0075] The step of irradiating may be completed in a period between 5and 75 nanoseconds.

[0076] The period may be between 10 and 50 nanoseconds.

[0077] Yet still further in carrying out the above object and otherobjects of the present invention, a system for thermal-based laserprocessing a multi-material device including a substrate and amicrostructure is provided. The system includes means for generating theat least one laser pulse having at least one predeterminedcharacteristic based on a differential thermal property of materials ofthe device. The system also includes means for irradiating themicrostructure with the at least one laser pulse wherein a first portionof the at least one pulse increases a difference in temperature betweenthe substrate and the microstructure, and a second portion of the atleast one pulse further increases the difference in temperature betweenthe substrate and the microstructure to process the multi-materialdevice without damaging the substrate.

[0078] The above object and other objects, features, and advantages ofthe present invention are readily apparent from the following detaileddescription of the best mode for carrying out the invention when takenin connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0079]FIG. 1a is a block diagram of a laser system which generates alaser pulse in response to a trigger signal obtained from a controlsystem, the pulse having a temporal shape including a fast rise and falltime, and a duration selected for the material processing application ofthe present invention;

[0080]FIGS. 1b and 1 c are views partially broken away illustrating amulti-layer, multi-material device wherein a laser pulse withpre-determined temporal and spatial characteristics irradiates thedevice;

[0081]FIG. 1b is a first side sectional view of a portion of the device,showing a target structure having a rectangular cross-section, wherein ahigh numerical aperture laser beam, having a non-unity aspect ratio, isincident on the target structure having a plurality of layers forming astack;

[0082]FIG. 1c is a second side sectional view of a portion of thedevice, orthogonal to the first, showing a rectangular target structure,wherein a high numerical aperture laser beam, having a non-unity aspectratio, is incident on the target structure;

[0083]FIG. 2a is a block diagram of a prior art laser system which showsa conventional q-switched or Gaussian pulse;

[0084]FIG. 2b is a view of a conventional multi-layer structure having asingle oxide layer between the link and substrate, therefore beinglocated in proximity to a substrate, with a conventional q-switchedlaser pulse irradiating and substantially overfilling the narrowdimension of the target structure;

[0085]FIG. 3 is a graph of reflection as a function of wavelength of amulti-layer stack having 28 layers in 14 pairs, the stack representativeof a device processed with a method and system of the present invention;

[0086]FIGS. 4a and 4 b are top views and associated graphs whichillustrate the effect of irradiating the target structure with laserbeam profiles of varying dimension with respect to the target structure;FIGS. 4a and 4 b show the result of truncating a representativenon-uniform Gaussian shaped laser spatial profile, wherein the energyenclosed by the target structure is strongly affected, the energy at thetarget edge varies, and potential stray radiation effects result fromenergy not absorbed by the target structure;

[0087]FIG. 4c is a side schematic view of a plurality of microstructuresformed on a layer and which illustrate that for decreasing spacing(pitch) inter-reflections and stray energy result in irradiation ofneighboring target structures;

[0088]FIGS. 5a and 5 b are graphs which show the reduction in irradianceon the device as a function of depth resulting from precise positioncontrol of a high numerical aperture beam (at the top surface), whereinthe position and depth of focus of the beam provides for processing ofthe target structure without creating undesirable changes to othermaterials; In particular,

[0089]FIG. 5a illustrates the increase in spot area with for variousspherical and elliptical Gaussian irradiance distributions, for arepresentative multi-layer stack used in a copper memory process;

[0090]FIG. 5b normalizes the defocus function relative to the energydensity (fluence) at the target location;

[0091]FIGS. 6a and 6 b are schematic views of a stack of layers formedon a wafer substrate and which illustrate exemplary results obtainedwith a ray trace simulation used to estimate the level of radiationimpinging on the internal layers and adjacent links with a specifiedbeam numerical aperture;

[0092]FIGS. 7a, 7 b, 8 and 9 are views of images taken from detectorsand which illustrate, on a continuous scale spanning 5 decades,simulated patterns of radiation at the surface, substrate, and with thestack removed respectively;

[0093]FIG. 10 is a schematic diagram of a system for measuring fiducialsor other alignment targets;

[0094]FIG. 11 is a graph of reflectivity versus outer layer thickness;

[0095]FIG. 12 shows a pair of graphs of reflectivity versus thickness ofthe outer oxide layer for two different laser beam wavelengths;

[0096]FIG. 13 is a schematic diagram of a system for automaticallycontrolling pulse energy based on a thickness measurement;

[0097]FIG. 14a shows schematic and graphical representations of aneffect of debris on signal fidelity during alignment measurements;

[0098]FIG. 14b shows similar representations with improved signalfidelity after cleaning with a pulsed laser beam;

[0099]FIGS. 15a-15 c show various arrangements for combining laserpulses or generating a sequence of closely spaced pulses using opticalor electronic delay methods;

[0100]FIG. 15a illustrates use of multiple lasers with delayedtriggering;

[0101]FIG. 15b illustrates a basic arrangement with a single laser andan optical delay path; and

[0102]FIG. 15c illustrates yet another modular optical delay lineproviding for pointing stability and simplified alignment;

[0103]FIG. 16 is a graph of temperature versus time which illustratessimulation results for metal link (top) and substrate (bottom)irradiance with a pair of delayed pulses wherein the substratetemperature decays rapidly exhibiting a differential thermal property ofthe materials; the two laser pulses each had a square temporal shape;

[0104]FIG. 17 is a series of schematic views of a metal link whichillustrate a multiple pulse sequence wherein: (1) a first pulseirradiates the metal link; (2) debris is left after removing the link;(3) a second pulse with a spatial pulse shape is used wherein thecentral zone is attenuated, the second pulse having a lower peak energydensity than the first pulse; and (4) 25 ns after the start of the firstpulse the debris is removed;

[0105]FIG. 18 is a block diagram of a system which generates andcontrollably selects pulses;

[0106]FIG. 19 is a block diagram of a system of the present inventionwherein a portion of a high repetition rate pulse train (e.g., 1 μHz) isselected and a high speed beam deflector (e.g., electro-optic oracousto-optic device) synchronized with microstructure positions is usedto process a single microstructure with multiple pulses during relativemotion; and

[0107]FIG. 20 is a block diagram of another system of the presentinvention wherein a beam deflector is used to spatially split a singlepulse so as to irradiate either one or two microstructures (or none)with a pair of pulses during relative motion.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0108] One aspect of the invention is removal of a microscopic targetstructure which is part of a multilayer, multimaterial device, whereinlaser energy is incident on several materials having dissimilar opticaland thermal properties. One application is memory repair. A newfabrication process (Damascene) includes a copper target structure,multiple dielectric layers in the form of a “stack,” and functionalcircuitry disposed at the dielectric layers. The target structure andlayers are typically formed on a silicon substrate. This is illustratedin FIGS. 1b and 1 c and corresponds to a device processed with anembodiment of the present invention. This will be referred to as a“multilevel” process.

[0109] With the use of more complex structures at finer scale (e.g., ator below a wavelength of visible light), considerations for reliableoperation of laser processing system increase to meet the standards forhigh yield in the semiconductor industry.

[0110] Aspects of the invention include methods and subsystems foroperation of the laser processing system. At the microscopic scale, thelaser beam waist diverges rapidly due to the small spot size and depthof focus. The materials within the 3D beam location may includefunctional circuitry. In an automatic system, robust measurement oftarget locations is used in conjunction with database information toposition a laser beam in three dimensions at high speed. The interactionof a laser beam within the multilevel device influences yield. Modelingof thermal interaction is useful of understanding and predictingperformance in the thermal processing regime. However, at themicroscopic scale, a more detailed understanding of interaction based onphysical optics is also beneficial.

[0111] In the following sections, detailed aspects of spatial andtemporal pulse shaping, three-dimensional measurement and prediction,device modeling and process design are disclosed with emphasis onsolving the problem of cleanly removing links on a multilevel device,wherein damage is avoided to inner layers and functional circuitrybetween a link and the substrate. However, various methods, subsystems,and experimental results may also be applied for link processing ofconventional single inner layer devices, and generally for processingmicrostructures surrounded by materials having dissimilar thermal oroptical properties.

[0112] Processing Links on a Multilevel Device

[0113] A pulsed laser beam, the beam having pre-determinedcharacteristics for processing of microscopic structures, is used tocleanly remove at least a portion of a target structure. An applicationof the method and system of the present invention is severing of highlyreflective copper links which are part of a high speed semiconductormemory device. The method and system of the present invention isparticularly advantageous for processing of targets having a sub-microndimension, including targets with a dimension below the wavelength ofthe laser beam. The target is separated from a semiconductor substrateby a multi-layer stack, which may have several dielectric layers.Furthermore, both the temporal and spatial characteristics of the pulsemay be selected or controlled based on the thermal and opticalproperties of the microscopic target, underlying layer materials, andthe three-dimensional layout of the device structure, including thespacing of target structures and functional inner conductor layers.

[0114]FIGS. 1a-1 c generally show an embodiment of the present inventionA laser pulse 3 irradiates a rectangular target structure ormicrostructure 10, side views of which are shown in FIGS. 1b and 1 c,with a focused beam. In a preferred embodiment, an output from shortpulse amplified laser system 1 is generated to produce the pulse 3 whichhas a rise time 4 fast enough to efficiently couple energy into a highlyreflective target structure. The duration 5 is sufficient to process thetarget structure wherein at least a portion of the structure is cleanlyremoved without leaving residue, slag, or other debris. The fall time 6is preferably fast enough to avoid creating undesirable damage to thelayers or substrate.

[0115] The temporal pulse shape is selected, in part, based on physicalproperties of the target microstructure 10, for instance, thickness,optical absorption, thermal conductivity, or a combination thereof. Inan advantageous embodiment of the invention, the processing will occurwith a single pulse having a fast edge leading relative to a selectedpulse duration of several nanoseconds. In an alternative embodiment, thelaser output may be a series of narrow q-switched or rectangular pulses,with very fast rise time, for example 800 ps pulses representative ofthe output of commercially available q-switch micro-lasers. The pulsesmay be delayed with respect to each other so as to provide a burst ofpulses to irradiate the target structure. The laser output may begenerated with a combination of a high bandwidth seed laser diode andfiber optic amplifier with Raman shifting, or with a waveguide amplifiersystem. Alternatively, a desirable pulse characteristic may be providedwith various modified q-switched systems or with the use of high speedelectro-optic modulators. Other pulse shapes may be selected for thematerial processing requirements. For instance, a sequence of closelyspaced pulses having duration from a few picoseconds to severalnanoseconds is taught in Reference 5.

[0116] In one embodiment, a high bandwidth MOPA configuration is used toamplify the laser output of a high speed semiconductor diode. Generationof various pulse shapes and duration with direct modulation of the diodeis considered advantageous, provided any affect associated with variableamplitude drive waveforms does not affect overall performance. Furtherdetails of various aspects of pulse generation and amplification can befound in references 5 and 6 (e.g., in '471—Reference 5—FIGS. 5 andcolumns 14-16).

[0117] As indicated above, embodiments of the laser system may includefiber optic amplifiers which amplify the preferred square pulse shapegenerated by a seed laser. The seed laser may be a high speedsemiconductor diode or the shaped output of a modified q-switchedsystem. The amplified output may be matched in wavelength to the inputor Raman-shifted as taught in References 4 and 6 (e.g., in Reference 6,FIGS. 12-13 and column 14, line 57—column 19, line 3). Wavelengthshifting of a short pulse q-switched laser output is generally taught in'759 Reference 4.

[0118] In an alternative arrangement the seed laser is a semiconductordiode and the optical amplifier is a waveguide amplifier. Advantages ofan embodiment with a waveguide amplifier when compared to a fiber systeminclude avoidance of Raman shifting, lower pulse distortion at the speedof operation, and, with proper design, minimal thermal lensing. Aprecision anamorphic optic system is used to optimize coupling betweenthe seed and amplifier. Basic description of waveguide amplitude andlasers can be found in product literature provided by Maxios, Inc. andin the article “CW and passively Q-switched Cladding Pumped PlanarWaveguide Lasers,” Beach et. al. Yet another amplifier system includinga 28 DB planar waveguide amplifier for use at 1.064 μm wavelengths wasdeveloped by University of Southhampton and described in “A DiodePumped, High Gain, PlanarWaveguide, Nd:Y3Al5O12 Amplifier.”

[0119] In an alternative arrangement, for generation of a fast risingpulse or other desirable shape, a plurality of q-switched micro-laserscan be used. The modules produce a q-switched waveform with pulsedurations of about 1 nanosecond or less, for example 800 ps to 2 ns forcommercially available units. An example of a commercially availablelaser is the AOT-YVO-1Q available from Advanced Optical Technology(AOTLasers.com). These recently developed short pulse, active q-switchlasers can be triggered with a TTL pulse at a variable repetition ratewhile maintaining specified sub-nanosecond timing jitter. In general,the pulse shape incident on the target microstructure will varysignificantly at repetition rates approaching the maximum rate.Reference 9 teaches methods of maintaining a constant pulse shapedespite variations in the temporal spacing of pulses incident on atarget (e.g., see the figures and associated specification). AOT offersa pulsewidth of 2 nanoseconds available at a repetition rate of 20 KHz.Frequency doubled versions are also available (532 nm). IMRA Americareports 800 ps pulses with the PicoLite system, and high peak power wasobtained with fiber amplification at repetition rates up to 10 KHz.Shorter pulsewidths, for instance about 1 ns or less, are available atslower repetition rates.

[0120] As known in the art and illustrated in Reference 5 (e.g., FIGS.1c, 2), the q-switched waveforms may approximate (at least to 1st order)a symmetric Gaussian shape, or a fast rising pulse with an exponentialtail, depending on the stored energy. With reference to FIGS. 15a-15 c,a series of devices, with appropriate delays introduced by a pluralityof triggering signals, or delays of a trigger signal with a delay line,is used to generate a series of spaced apart pulses. The optical outputsare preferably combined with appropriate bulk optics (polarizationsensitive), fiber optics, or waveguides to form a single output beam.The resultant addition of the q-switched waveforms produces a fast risetime characteristic and relatively short duration. An optical amplifier122 may be used to increase the output power as needed.

[0121]FIG. 15a shows a schematic of one basic embodiment with bulkoptics, where a beam combiner 123 is used to deliver the output of twolasers 120,121 to an amplifier 122. A delay circuit 126, which may beprogrammable, controls triggering. Polarization optics 127,128 are usedto provide the proper input to the beam combiner. In one arrangement thepulses are spaced apart and appear as a high frequency burst 124. In asecond arrangement triggering of the second pulse occurs at a slightlydelayed (but controlled) position which produces a characteristicapproximating a square pulse shape 125. In the latter arrangement thecontrolled delay is about 50% of the FWHM. Those skilled in the art willrecognize that alternative arrangements may be used with multipleamplifiers, combiners, with bulk, fiber, or integrated opticarrangements.

[0122] Generation of multiple pulse waveforms may also include some formof active q-switching of two separate microlasers or detecting a firstpulse from a passively q-switched laser and subsequently triggering anactively q-switched laser or MOPA relative to the first pulse.

[0123]FIG. 15-b is a basic schematic showing the use of a single laser140 wherein the laser output is divided by beam splitter 142, whereby aportion of the beam propagates along a path 141, followed by combiningwith combiner 143, after polarization adjustment with rotator 146 whichmay be a half-wave plate. An optional optical amplifier 145 may then beused to produce higher output power.

[0124] In an arrangement using a single laser and an optical delay line,the optical system will preferably be stable and easy to align. FIG. 15cshows an exemplary embodiment wherein the use of opposing corner cuberetroreflectors 130 makes the setup insensitive to tilt of the foldingelements. The angular alignment of the delayed beam paths 131,132 isvery stable even in a high vibration environment. One of the cornercubes in each pair of retroreflectors 130 is initially adjusted in theX/Y translation and Z rotation to get the transverse position of thedelayed beam path centered. Each of the λ/2 retarders 133 in the mainbeam path is adjusted so that vertical or horizontally polarized lightwill have its polarization rotated by 45 degrees. The λ/2 retarder 133in the second delay loop is adjusted so that vertical or horizontalpolarized light will have the polarization rotated by 90 degrees causingthe delayed pulse in the second loop to circulate twice before exiting.The peak-to-peak spacing of the output waveform 135 (e.g., 4 combinedpulses) is controlled by the length of the delay loops. If non-equalamplitudes for the delayed pulses are desired, the λ/2 retarders 133 inthe main beam can be set for a polarization of other than 45 degrees.Likewise, the pulse shape can be varied at the time a system is setup orpossibly in operation by manually or automatically controlling thespacing. Those skilled in the art of laser pulse generation and shapingwill appreciate the advantages of the compact and modular arrangementfor short pulse for typical delays ranging from a few nanoseconds totens of nanoseconds. For instance, U.S. Pat. No. 5,293,389 to Hitachidescribes a polarization-based fiber delay line for generating laserpulses of decreasing amplitude for generating longer pulses, forinstance 100 ns or longer.

[0125] Another means of producing a shaped pulse is to use the modulatorapproach to chop the leading edge or tail of the pulse but with atwo-stage or shaped modulation voltage pulse. For example: with a 10 nsq-switched pulse, the modulator could have 100% transmission for thefirst 1-5 ns followed by 25% transmission for the remainder of thepulse. Early pioneering work by Koechner (U.S. Pat. No. 3,747,019) andSmart (U.S. Pat. No. 4,483,005) demonstrate exemplary amplitude andpulse shape control methods using electro-optic modulators.

[0126] The multiple pulses shown in FIGS. 15a-15 c may or may not havethe same wavelength, and the temporal shape of a pulse may be varieddepending upon specific requirements. For example, in certainembodiments an output may be a q-switched pulse of short duration andhigh peak power combined with a lower power square pulse shape.

[0127] Referring to FIGS. 1a and 1 b, during system operation for memoryrepair, position information, obtained with a precision measurementsystem, is used to relatively position the focused beam waist of thepulsed laser at a location in space 7,8,9 to substantially coincide withthe target 10 three-dimensional coordinates (Xlink,Ylink,Zlink). Atrigger pulse 2, generated at a time where the laser beam waist andtarget position substantially coincide, operates in conjunction with thelaser and associated control circuitry in laser subsystem 1 to producean output pulse.

[0128] References 2 and 7 describe details of a method and system forprecision positioning, including three-dimensional beam waistpositioning. Reference 7 describes a preferred embodiment for producingan approximate diffraction limited spot size with a range of spot sizeadjustment (e.g., FIGS. 7-9of WO0187534 ('534) and the associatedspecification), and a preferred method and system for three-dimensionalpositioning of the beam waist. Three-dimensional (height) information isobtained, for instance with focus detection, and used to estimate asurface and generate a trajectory (e.g., FIGS. 2-5 of '534 and theassociated specification). The laser is pulsed at a locationsubstantially corresponding to the three-dimensional position of thelink (Xlink, Ylink, Zlink) (e.g., FIGS. 10a-b of '534 and the associatedspecification).

[0129] In practice, the three-dimensional measurement and positioningare used to compensate for topographical variations over a wafersurface, or other position variations introduced in a system(mis-alignment). These variations are generally system or applicationdependent and may exceed several microns, which in turn exceeds thedepth of focus of the focused laser beam. In some micro-machiningapplications the system positioning requirements may be relaxed ifcertain tolerances are maintained, or if external hardware manipulatesthe device position, as might be done with a micro-positioningsub-system. The device may comprise a miniature part (e.g., single die)which is positioned by an external micro-positioning subsystem to apredetermined reference location. Similarly, if a miniature part has apre-determined tolerance the positioning may be based on singlemeasurement at a reference location or perhaps a single depthmeasurement combined with a lateral (X,Y) measurement. For processing ofmultilevel devices on wafers, (e.g.: 300 mm) at high speed it isexpected that densely sampled three-dimensional information will improveperformance, particularly as link dimensions shrink.

[0130] In applications requiring very high speed operation over a largesurface (e.g., 300 mm wafer), an alternative method is to combineinformation which may be predetermined (e.g., the plane of a wafer chuckrelative to a beam positioner plane of motion measured during acalibration process) with dimensional information obtained from eachpart to be processed. For example, in '534, FIGS. 1-2, a fraction of thetilt of region 28 may be associated with fixturing). For example, thesteps may include (a) obtaining information identifying microstructuresdesignated for removal, (b) measuring a first set of reference locationsto obtain three-dimensional reference data, (c) generating a trajectorybased on at least the three-dimensional reference data to obtain aprediction of beam waist and microstructure surface locations, (d)updating the prediction during relative motion based on updated positioninformation, the updated position information obtained from a positionsensor (e.g., encoder) and/or from data acquired during the relativemotion. The additional data may be measurement data acquired atadditional alignment target or at other locations suitable for anoptical measurement (e.g., dynamic focus). Reference 2 describes asystem wherein a precision wafer stage is used to position a wafer athigh speed. A method of obtaining feedback information with resolutionof a fraction of one nanometer is disclosed wherein interferometricencoders are used, and such a high precision method is preferred. InReference 2 it was noted that other conventional laser interferometersmay also be used. FIGS. 9-11 and columns 5-6 of Reference 2 describeaspects of the precision measurement subsystem associated with theprecision positioning apparatus. Additionally, designated referencelocations on the workpiece (e.g., wafer) which may be an x,y alignmenttarget or a region suited for a three-dimensional measurement may beused for various applications. It should also be noted that heightaccuracy of about 0.1 μm was reported in “In-situ height correction forlaser scanning of semiconductor wafers,” Nikoonhad et al., OpticalEngineering, Vol. 34, No. 10, October 1995, wherein an optical positionsensor obtained area averaged height data at high speeds. Similarly, adynamic focus sensor (e.g., astigmatic systems used for optical disktracking and control) may be used to obtain height information providedthe data rate is fast enough to support “on the fly” measurement.

[0131] Various combinations of the above technologies can be useddepending upon the application requirements. A combination may be basedon the number and typical distribution over a device of microstructuresdesignated for removal. When a large number of repair sites aredistributed across a device, the throughput may be maximized byproviding updates “on the fly.”

[0132] In an application of the invention, the target structure 10 isprovided as a part of a multi-material, multi-layer structure (e.g.,redundant memory device). The multi-layer stack having dielectric layers14,15 provides spacing between the link and an underlying substrate 17.In one type of multi-layer memory device, alternating layers of SiliconDioxide 15 and Silicon Nitride 14 may be disposed between a copper linktarget structure 10 and a Silicon substrate 17. The copper targetstructure is generally located in proximity to other similar structuresto form a 1-D or 2-D array of fuses which are designated for removal. Inaddition to the copper link structure, underlying conductors 16 disposedas part of the functional device circuitry, may be in proximity to thelink structure, and arranged in a series of patterns covered byrelatively thin (<0.1 μm typical) Silicon Nitride 14 and thicker (1 μmtypical) Silicon Dioxide 15 materials.

[0133] The irradiance distribution at the link may substantially conformto a diffraction limited, circular Gaussian profile. In another usefulembodiment, the beam has an approximate elliptical Gaussian irradianceprofile, as might be produced with an anamorphic optical system, or witha non-circular laser output beam. In one embodiment, the incident beamhas a non-uniform aspect ratio 12,11 as also illustrated in FIG. 4b(e.g., 3:1). Alternatively, rectangular or another chosen spatialprofiles may be implemented in a lateral dimension. For example,Reference 1 discloses various advantageous methods and optical systemsfor “non-Gaussian” spatially shaping of laser beams for application tomemory repair.

[0134] With the nearly diffraction limited elliptical Gaussian case, thepreferable minimum beam waist dimension at location 11 approximates thenarrow target 10 dimension of FIG. 1b, which, in turn, produces highpulse energy density at the link. Further, with this approach, a highfraction of the laser energy is coupled to the link and backgroundirradiance is reduced.

[0135] A typical copper link used in a present memory has width andthickness of about 1 μm or less, for example, 0.6 μm, and length ofabout five microns. Future memory requirements are expected to furtherreduce the scale of target dimensions. The minimum beam waist dimensionWyo at 11 will typically overfill the sub-micron link to some degree,whereas aspect ratio Wxo/Wyo 12,11 with Wxo a few microns along thelink, can facilitate clean link removal. Additionally, rapidlydecreasing energy density on the layers 14,15 and substrate 17 isachieved through defocus of the high numerical aperture beam portion 11.

[0136] The graphs of FIGS. 5a and 5 b illustrate the estimated defocusfor various aspect ratios, relative to a circular Gaussian and anelliptical beam at best focus. FIG. 5a shows the very rapid falloff of a1.6 μm circular Gaussian (0.002 mm numerical divisions =2 μm). FIG. 5bshows a normalized result to scale the energy density at best focus forthe different spot shapes. These results indicate that with precisionbeam positioning in depth, wherein the power density is maximized at thetarget site, at relative reduction in energy density of more than onedecade occurs at the substrate level for an exemplary multi-layer stackused in a copper based process for memory fabrication. Further, therapid defocus relative to the waist WyO is beneficial for avoiding innerlayer damage, provided the “tails” of the incident beam irradiatefunctional inner layer 16 (e.g., copper) at a low level.

[0137] In one embodiment for processing a multilevel device, copper linkremoval is initiated with application of the fast rise time pulse,having a nominal 10-90% rise time 4 in a preferred range of less than 1nanosecond to about 2 nanoseconds. A pulse duration 5 in the range ofabout 2 nanoseconds to 10 nanoseconds is preferable to sever the linkwhile limiting thermal diffusion. Pulse energies in the range of about0.1 microjoules (μj) to 3 μj were effective, with a preferred typicalrange of about 0.1-5 /μj considered sufficient margin for spot shape andprocess variations. The preferred pulse duration may be selected basedupon the nominal link thickness specifications, or based on a model ofthe dissimilar thermal and optical properties of adjacent materials.During the pulse duration, thermal shock of top layer 13 and thermalexpansion of the target 10 result in explosion of the link throughruptured top oxide layers 13, which in turn reduces the stress at thelower corner of the link structure adjacent to the layer 14. The laserpulse is rapidly terminated, preferably within a few nanosecond falltime 6 after the explosion, at a time just after the thin link iscleanly severed, and prior to a time the lower corner of the linkresults in cracking of at least layer 14. Further details and resultsrelated to the interaction of a laser pulse with a metal link andoverlying layers is disclosed in references 4 and 5. The '471 patent andthe associated specification describe the interaction process (e.g.,FIGS. 1a , 1b, 11a, 11b, and in column 18).

[0138] Hence, a combination of the spatial characteristics (e.g., beamwaist shape and position) and the temporal (e.g., rise time 4, flatness,and duration 5) pulse characteristics avoids undesirable cracking oflower layers 14,15, avoids significant pulse interaction with innerlayer conductor 16, and limits substrate 17 heating. Hence, despite thehigh reflectivity of the copper link at visible and near infraredwavelengths, and the expectation in the prior art of incomplete removaland damage to surrounding structures and substrate, the target structureis processed without undesirable damage to other structures. It is alsoknown that copper, in addition to having nearly maximum reflectance inthe near IR, is also more reflective than other link materials (e.g.,aluminum, platinum). Nevertheless, due to the optical interaction of thenear IR beam with the target and the optical and thermal properties ofadjacent (overlying) layers, the preferred copper material can beprocessed.

[0139] Furthermore, near IR (Infrared) wavelengths also convenientlycorrespond to wavelengths where high bandwidth laser diodes areavailable, and to the spectral range where optical amplification of thepulsed laser beam can be efficiently produced with fiber and waveguideamplifiers. Those skilled in the art will recognize that amplified laserdiode outputs, having a desired temporal pulse shape, may also befrequency multiplied to produce visible laser outputs when advantageous.The fast rise time of semiconductor diodes is particularly advantageousfor producing a fast rise time, square pulse characteristic. Futuredevelopments in visible diode and optical amplifier technology maysupport direct pulse amplification in the visible range.

[0140] In a preferred system for copper link blowing, the link width isa fraction of one micron and the link spacing (pitch) is a few micronswith present process technology. The link width may typically correspondto a wavelength of visible light. Further, at the microscopic scale ofoperation, where the lateral and/or thickness dimensions of thematerials of FIGS. 1b and 1 c are on the order of the laser wavelength,the thickness and indices of refraction of the stack materials cansignificantly affect the overall optical characteristics of the stack.

[0141] In one embodiment of the invention, a preferred reducedwavelength is selected in the visible or near infrared range wherein anon-absorptive optical property of the layers (e.g., interference orreflection loss) is exploited. The device structure of FIGS. 1a and 1 bcan be damaged with substantial absorption within the lower layers, suchdamage is prohibitive because of the presence of adjacent circuitry.This is in contrast to link processing with the prior art system of FIG.2b where inner layer damage is not generally detrimental to overalldevice performance.

[0142] U.S. Pat. No. 6,300,690 (Reference 8) describes a system andmethod for vaporizing a target structure on a substrate. The methodincludes providing a laser system configured to produce a laser outputat the wavelength below an absorption edge of the substrate.Furthermore, Reference 4 discloses benefits of a wavelength less than1.2 um for processing links on memory devices wherein the substrate isSilicon, namely smaller spot size and shorter laser pulsewidths. Inaccordance with the present invention, improved performance can berealized by exploiting the non-absorbing stack properties withwavelength selection. Furthermore, at least one of precision positioningof a high numerical aperture beam, spatial shaping of the spot, ortemporal pulse shaping also will provide for reduced energy at thesubstrate. The result corresponds to a relatively low value of energyexpected to be deposited in the substrate, despite an incident beamenergy necessary to deposit unit energy in the target structuresufficient to vaporize the target structure.

[0143] The factors affecting the energy deposited in the substrate are,in effect, multiplicative. Likewise, at short visible wavelengths,copper is absorbing (e.g., about 50% at 500 nm, 70% in the near UV,compared to 2% at 1.064 um) so less energy is required for cleanremoval, at least an order of magnitude. The preferred identifiedwavelength corresponding to a relatively low value of the energyexpected to be deposited in the substrate is within a visible of near IRregion of the spectrum. A model-based approach may be used to estimatethe shortest wavelength with sufficient margin for a specifieddielectric stack, spot position, tolerance, temporal andthree-dimensional spatial pulse characteristics.

[0144] For processing on links on multilevel devices with Siliconsubstrates, the limiting wavelength corresponding to a relatively lowvalue of the energy expected to be deposited in the substrate (e.g.,below the image threshold) may be within the green or near UV region ofspectrum, but the use may require tightly controlled system parameters,including possible control of the stack layer thickness or index ofrefraction.

[0145] With wavelength selection in accordance with the presentinvention, where the internal transmission and preferably reflection ofthe stack is at or near a maximum, stack layer damage is avoided.Furthermore, decreasing substrate irradiance, while simultaneouslyproviding a reduced spot size for link removal (at or near diffractionlimit), is preferred provided irradiation of functional internal layersis within acceptable limits. Spectral transmission curves for typicallarge bandgap dielectric materials generally show that the transmissiondecreases somewhat at UV wavelengths. For example, in HANDBOOK OF LASERSCIENCE AND TECHNOLOGY, the transmission range of Silicon Dioxide isspecified as wavelengths greater than 0.15 μm. The absorptioncoefficient of both Silicon Nitride and Silicon Dioxide remainsrelatively low in the visible range (>400 nm) and gradually increases inthe UV range.

[0146]FIG. 3 is a graph which illustrates the estimated back reflectionproduced by a representative multi-layer stack of 14 Silicon Dioxide 15and Silicon Nitride 14 pairs over a range of near IR wavelengths, wherethe thickness of the layers is about 1 μm and 0.07 microns,respectively. In accordance with the present invention, a large numberof layers can be accommodated, and may range from about 4-28 dependentupon the process (e.g., sometimes multiple layers may separate afunctional conductor layer).

[0147] By way of example, it is shown that significant reflection occursover relatively broad wavelength range. A single layer disposed as aninternal layer 14 will typically reflect roughly 2% at each surface overthe visible and near IR spectrum. It is well known in the art of linkand semiconductor processing that Silicon absorption varies by orders ofmagnitude in the near IR spectral range. Further, studies of Siliconmaterial processing have shown that the absorption is unstable andnon-linear with increased laser power and substrate heating atwavelengths near the absorption edge, as taught in reference 4. However,as stated above, the shorter wavelengths are preferred to producesmaller spots (references 4-6, and 8) and higher energy concentration atthe link position.

[0148] In accordance with the present invention, exploiting the layerreflection with wavelength can further enhance the system performanceand supplement the benefits associated with temporal and spatial controlof the pulse in a preferred short wavelength range. Such wavelengthselection is regarded as particularly advantageous at wavelengths wherethe substrate absorption would otherwise greatly increase, andsignificant margin can be obtained when the number of layers 14,15disposed between the link and substrate substantially exceeds the numberof overlying layers 13. A preferred structure for processing willcomprise a substantial number of layers, with large reflectance at apredetermined short wavelength, the wavelength being well matched forgeneration of the preferred fast square temporal pulse shape.

[0149] Standard laser wavelengths in the range of FIG. 3 include 1.047μm and 1.064 μm, the latter being a standard wavelength of semiconductordiodes. Further, custom wavelengths include 1.08 μm, and otherwavelengths generated with Raman shifting. Those skilled in the art willrecognize that frequency multiplication of the near IR wavelengths canbe used to generate short wavelengths, and with appropriate designmultiple wavelengths may be provided in a single system. For instance apreferred temporal pulse shape, with a fast rise time, may be generatedin the green portion of the visible spectrum by frequency doubling anear IR laser.

[0150] In an alternative embodiment, wavelength tuning is used to matchthe wavelength to the approximate peak reflectance of the stack. Such anarrangement may be particularly advantageous for adjustment of a laserwavelength at the edge of the reflectance range (i.e., “cutoff” range)over a limited wavelength range, whereby sensitivity to tolerances inthe material thickness and index of refraction are avoided. As notedabove, further discussion of laser amplifier systems and application toother link structures can be found in references 4-6.

[0151] Generation of the pulsed laser beam may include the step ofshifting the wavelength of the laser beam from a first wavelength to apredetermined wavelength. The predetermined wavelength may be based onmaterial characteristics comprising at least one of: (1) couplingcharacteristics of the microstructure, (2) multi-layer interference, and(3) substrate reflectivity.

[0152] Experimental results have shown that at a wavelength of 1.047 μm,where the absorption of Silicon in orders of magnitude higher than at1.2 μm, substrate damage is avoided with a short q-switched (standard)pulse and the stack characteristic of FIG. 3. However, the results witha standard laser having a q-switched temporal pulse shape showedcracking of an oxide layer 14 below the link. The relatively slow risingq-switched pulse shape, which for a Gaussian approximation is asubstantial fraction of the duration, was considered a limiting factorfor link removal without cracking of the inner layer based onexperimental results. However, based on the teachings of the prior art,severe damage to the Silicon substrate would be expected at the 1.047 μmwavelength because the absorption is orders of magnitude higher than ata wavelength corresponding to maximum transmission. In accordance withthe teachings of the present invention, the spatial pulsecharacteristics and the stack reflection are important factors toconsider so as to avoid inner layer and substrate damage and shortwavelengths of operation (which also provide for a smaller spot size andhigher energy concentration at the link). Further, in accordance withthe present invention, a predetermined square pulse shape generated at alaser wavelength of 1.047 μm would be expected to produce clean removalwithout undesirable changes to the stack and substrate.

[0153] Laser Processing and Process Design at the Sub-Micron Scale

[0154] Furthermore, in an exemplary advantageous embodiment for shortwavelength processing of reflective microscopic structures, aspecification for a multi-layer stack may be considered in processdesign. For example, a quarter-wave stack of alternating dielectrics orother suitable arrangement having a large difference in the index ofrefraction, and high transmission within each layer, is specified at aselected wavelength. It can be shown that very high reflectance isachievable, the quarter-wave stack being easily computed in closed formand modeled. Hence, the method and system of the present invention canbe used effectively with other aspects of process design, and may beadvantageous where the absorption of deeply buried layers and thesubstrate is relatively high, or where the width of a target structureis well below the laser wavelength.

[0155] The design of the device structure may have certain constraintsrelated to the layout of the circuitry. As such, certain thickness andmaterial for a certain layer may be defined, for instance an insulatorin a plane of a conductor having the approximate thickness of theconductor, or related to the thickness of the conductor. It may bepossible to select a material having a different index of refractionthan the specified layer. A specified thickness may be based on theestimated reflection at an advantageous laser wavelength which mayreduce or eliminate a requirement for special laser equipment operatingat “exotic” wavelengths where the lasers are difficult to manufacturewith high yield. The reflection may be estimated using a model whereinthe thickness is a variable, and an estimate made to maximize thereflection, subject to other device constraints.

[0156] Thickness of the layers can be tuned to a wavelength in as muchas the wavelength (or angle) can be tuned to the layers. Index ofrefraction could be used to fine-tune over a limited range, but therange may not be significant for small changes in index. Even with allthicknesses fixed by the process, the addition of a variable thicknesstuning layer or layers with predetermined thickness could be used tosignificantly affect reflectivity of the whole stack. For example, alayer not constrained by metallization requirements could be used as aprecision spacer between an upper and a lower stack portion. This couldbe a very powerful tool for tuning the process with adjustment ofperhaps only one layer.

[0157] Physical Optics and Laser Processing of Multi-Level Devices

[0158] Other controllable laser characteristics may be exploited,alternatively or in conjunction with wavelength selection, to providefurther improvements in the processing energy window. Reference 3describes an advantageous method and system for polarization control,including dynamic polarization selection and computer control so as toalign the polarization with a link orientation (e.g., details shown inFIG. 4 and the associated description in the reference). Thepolarization can be selected on the basis of the target couplingcharacteristics, the film reflectance, or a combination thereof.

[0159] With a link dimension below the spot size, effects likediffraction, scattering, and edge reflection should be considered asphysical phenomena which can have either advantageous or detrimentalresults depending upon the device geometry and beam characteristics.Likewise, at high energy density, non-linear absorption may affectresults, with particular concern of semiconductor material damage.

[0160] An additional important consideration with fine pitch (spacing)of adjacent links and circuitry is collateral damage. Furthermore,functional circuitry in a plane of the layers must not be damaged. Withan increasing trend toward fine pitch and high density memory, thethree-dimensional structure of the device should be considered and mayaffect a choice of beam spatial and temporal characteristics. By way ofexample, FIGS. 4a-4 c illustrate effects of reflection and diffractionassociated with sub-micron width link 10 resulting in truncated 43,44Gaussian beams 11, where the spot size (as measured at the 13.5% point)is wider than the link by varying degrees. The sketches arerepresentative of a diffraction limited beam waist at a near IRwavelengths. The central lobe is clipped by the link, which appears as anear field obscuration, resulting in transmitted beam portions which aretruncated 43,44. Energy which is not incident on the link may propagateat wide angles into the layers 49 which may be advantageous from thestandpoint of avoiding damage to the substrate 17 as shown in FIG. 1. Inany case, there will be some correlation of neighbor irradiance withspot size. Large spots with a relatively large depth of focus havereduced divergence and neighbor irradiance can be small, provided thatthe link spacing is large enough that the non-absorbed energy 43 of theincident beam impinging on an adjacent structure is weak, for instancecorresponding to level 44. With a higher N.A. and smaller spot size, thereflected beam diameter at the link location 46 is increased. There willbe a maximum value for some spot size 41,42. Then irradiance at aneighboring link 48 decreases as the reflected energy grows larger inarea.

[0161] Simultaneously there is an angular variation in internalreflection. Hence, the stack layer thickness can also effect theirradiance of adjacent structures, including the internal structures 16of FIG. 1. Furthermore, polarization variations with angle are expectedto produce variations. FIGS. 6a and 6 b illustrate by way of examplegeometric ray tracing effects of internal reflections propagating overan extended area.

[0162] Similarly, as shown in FIG. 4c, if a portion 45 of the laser beamincident on the edge of the link 46 is considered, the energy which isnot coupled into the link structure may also be scattered and/orspecularly reflected to the adjacent links 48. The inter-reflection 47occurs as a consequence of at least the link 46 physical edge profileswhich may be slightly curved or sloped.

[0163] An additional consideration is the three-dimensional spacingbetween an inner conductor layer 16 of FIG. 1, the beam waist 11, andthe adjacent links 48 of FIG. 4c. A large numerical aperture beam waist11, producing the smallest spot size at the link, while diverging andreflecting in a manner so as to avoid significant interaction with theinner layer 16 is preferred. Examination of FIGS. 4a-4 c suggest areduced spot size with controlled precision 3D waist positioning isexpected to reduce collateral damage by maximizing energy coupled intothe link. With high enclosed energy within the link and a low intensitytransmitted profile 44, edge reflection is minimized. The spatialprofile should also be selected subject to the constraint of only lowlevel, negligible interaction between the beam angular distribution at16.

[0164] It is preferred that the interaction mechanisms associated with aportion of the three-dimensional device structure be modeled forselection of at least a spatial pulse characteristic, such acharacteristic may be the N.A. and position of the beam waist.Preferably, the model will include an estimate of the irradiance seen byeach adjacent link structures 48, internal layer 16, and substrate 17.Whereas damage to adjacent link structures may be relatively apparentwith conventional microscopy, assessment of inner layer 16 and substrate17 damage may be considerably more difficult with the 3-D devicestructure.

[0165] With link widths below 1 μm, and pitch of a few microns, precise,sub-micron alignment is required to compensate for variations betweenwafers, and local variations within a wafer, and system tolerances(e.g., 300 mm wafer with 25 μm of topographical variation, and 5 μm ofmanufacturing tolerances, for instance). In accordance with the presentinvention, a precision positioning method and system is used torelatively position the beam waist so as to provide high laser energyconcentration at the link. Also, one important consideration forprecision positioning is predicting accurate (Xlink,Ylink) locationinformation. The prediction is subsequently used by a motion control andpositioning system to generate a laser output via trigger 2 at thetarget coordinates, during relative motion of the target 10 and laserbeam. A preferred embodiment includes a polarization insensitivescanning and detection system as described hereinbelow, wherein a regioncontaining an alignment target location is imaged to obtain referencedata. The target location is often covered by a dielectric layer ofSilicon Dioxide, Silicon Nitride, or other insulating material.Experiments have indicated that polarization insensitive detection isadvantageous to avoid spurious measurements. The results led to ahypothesis that that birefringence is introduced in the insulatinglayers by polishing or other process operations, which is manifested bypolarization variations in the reflected beam. These variations reducethe signal-to-noise ratio and appear to induce position distortion. Thedigital output data from each target location is used by an 8-parameterleast squares alignment algorithm to estimate and correct positioninformation affected by offset, angle, scale, orthogonality, andtrapezoidal variations over the wafer containing the links to beprocessed.

[0166] Given the variations in the received beam at the target location,concerns arise that process variations may affect layer opticalproperties near the target structure. Furthermore, in practice,variations occur in the thickness and reflectivity of the target andlayers, either over a wafer to be processed or from batch-to-batch.Measurement of the thickness and reflectivity is useful for processmonitoring, and can also be used to determine adjustments for the laserpower and wavelength to increase the energy window. For instance, anyvariation in the reflectivity of the link can affect the energy requiredfor processing. A preferred method and system for adaptive energycontrol is also described hereinbelow.

[0167] As dimensions of links and other microscopic structures continueto rapidly shrink, those skilled in the art will appreciate the benefitsof multi-parameter modeling. A model-based approach leads to selectionand precision control of the spatial and temporal characteristics of thelaser output, resulting in controlled three-dimensional interaction ofthe laser with complex multi-layer, multi-material structures.

[0168] Polarization Insensitive Detection and X,Y Reference Measurements

[0169] Commercial laser systems of the assignee of the present inventionuse a beamsplitter to pick off a portion of the reflected light from thework surface (e.g., a multi-layer memory device) as the laser isrelatively positioned 152 over the alignment targets (e.g., fiducials).A block diagram of the subsystem is shown in FIG. 10. Thereflectance/transmission (RIT) split of the beamsplitter 150 depends onthe laser that is being used. In cases where the laser has low totalenergy and as much transmission as needed is necessary, the split of 90%transmission and 10% reflectance is made. This gives 90% going to thework surface on the way in and the 10% reflected is dumped. But thisonly picks off 10% of the reflected light, 90% of the reflected light istransmitted back down the laser path. When possible, the split 70/30 ismade. This gives less total energy to the work surface but gives higherreflected signal.

[0170] Regardless of the R/T split, the specification is the R/T for Spolarization=R/T for P polarization (within 5%). This is accomplishedwith a special dichroic coating, which produced good results. Becauseany polarization state can be thought of as a vector sum of S and P, thebeamsplitter works at the correct R/T ratio for any polarization.

[0171] This is important because switching polarization to any desiredstate is done in the preferred link processing system to improve linkcutting efficiency. For example, co-pending U.S. application Serial No.01/013,956, filed Dec. 13, 2001, a continuing application of U.S. Pat.No. 6,181,728 (Reference 3) and assigned to the assignee of the presentinvention reports results wherein a process window improvement occurswith polarization perpendicular to the link, particularly as the spotsize is reduced. The preferred polarization controller disclosed in the'728 patent is used to switch states.

[0172] The method and system of the present invention are advantageouswhen there are oxide layers over the targets to be scanned and measured.The oxide layer may affect the polarization of the beam. This may happenbecause the oxide layer is stressed and creates birefringence. With thepolarization insensitive arrangement this is not a problem, no matterhow the polarization is changed one gets the same reflectance from thebeamsplitter and the same signal level. If a more typical polarizingbeamsplitter or simpler coating is used for the beamsplitter, thechanged polarization will result in a change in the reflected signal. Ifthe stress in the oxide layer varies, especially where it is over thetarget microstructure (it may be stressed because it is going over atarget edge) then the polarization may vary as the beam scans thetarget. Again, this is not a problem because of the coating. In thepolarizing beamsplitter case the reflected signal 151 measured at thedetector would vary because the polarization is varying at the same timethat one is trying to gather edge data, skewing the resultsuncontrollably and unpredictably.

[0173] This polarization insensitive technique is regarded as the mostrobust method and is preferred for measuring targets covered by at leastone oxide layer. However, other imaging and edge location methods may beused, but may require more complex measurement algorithms to accuratelymeasure the targets in the presence of multiplicative image noise.

[0174] Measurement with Anomalous Reflectivity Variations—Cleaning witha Pulsed Laser Beam

[0175] A typical alignment target 100 is depicted in the schematicdrawings of FIGS. 14a and 14 b. The target 100 is typically covered withone or more passivation layers, these may correspond to the layer 13 inFIGS. 1b and 1 c, but are not so restricted. During experiments withlink removal on a multi-level, the preferred polarization insensitivemeasurement method-was used to obtain X,Y target locations. However, itwas discovered that debris 1001 within the target area 100, possiblyfrom residual solder flux from nearby solder deposits (solder balls),significantly affected the reflected signals obtained with a detectorresulting in noisy profiles 101. The impact on the measurement wasmanifested as a large residual in the least squares fit algorithm usedto estimate location. In this illustration, the target area is shown asa positive contrast (e.g., higher measured intensity) region, but thoseskilled in the art will recognize that contrast reversal is acceptableprovided that the contrast between the target 100 and the background isadequate for measurement.

[0176] A pulsed beam with lower peak power was used to remove thedebris. An enhanced exemplary signal profile 102 (e.g., associated withrelatively uniform intensity and a mostly debris-fee region) wasobtained as a result of the cleaning operation, as shown in FIG. 14b.Representative energies for cleaning were on the order of 0.01 μj, forinstance, 0.005 μj. This is well below the damage threshold of thematerials, and well below the typical energies used for removal of links12.

[0177] In one embodiment, a single linear scan or a plurality of linearscans 104 across the target 100 are used to obtain reflected intensitydata which is analyzed statistically to measure fidelity, for instanceby determining the % intensity variation or standard deviation. In anexemplary embodiment, data is taken along the line(s) 104 at about every0.001″. However, the sample space may be finer or coarser depending uponthe signal fidelity obtained. If the spacing is too fine, additional“texture noise” may be introduced. If too coarse, an edge contrast 107will be reduced or errors introduced by undersampling. If the variationis excessive, a cleaning operation is initiated with a pulsed beam.Preferably, the laser power is controlled with an acousto-opticmodulator (i.e., “energy control” of FIG. 13) which is a standard partof the laser processing equipment. The operation of the modulator forintensity control and pulse selection within a link blowing system isdescribed in more detail in U.S. Pat. No. 5,998,759 (e.g., Reference 4,col. 7, and the associated drawings). Those skilled in the art willrecognize that such modulators provide for intensity control over a widedynamic range, e.g., 100:1. A relatively simple user interface canprovide for operator interaction to initiate operation, based on“pass/fail” or other criteria.

[0178] In another embodiment, the linear scan(s) may be doneautomatically and the cleaning operation performed at each measurementlocation.

[0179] In a preferred arrangement, only an adjustment of the energy willbe needed, and other system parameters unaltered during the cleaningoperation or as a result of cleaning. Those skilled in the art ofmeasurement will be able to make various adjustments in the systemparameters based on correlation of the results with other processparameters.

[0180] In a preferred arrangement, the cleaning operation will beapplied only to scanned regions as needed. In one arrangement, theprocess is iterative with a measurement goal of obtaining suitableresiduals in the least squares fit algorithm. If the residuals are abovea designated value, scans of at least one region are obtained andcleaning occurs. In some cases, it may be desirable to adjust thepositions of the scan lines (e.g., if cleaning is difficult). A fidelitymeasurement (e.g., contrast, standard deviation) may be used to guidethe cleaning operation. Preferably, no more than one pass will berequired.

[0181] It is to be understood that numerous arrangements could be usedto practice the cleaning invention. For instance, an array camera couldbe used with different wavelength illumination to identify regions ofnon-uniform intensity. These regions could then be designated forcleaning. Those skilled in the art of optical measurement will be ableto implement such arrangements, and such arrangements are within thescope of the present invention.

[0182] Reflectivity Measurement and Power Adjustment—Case 1: SingleWavelength

[0183] The above discussion related to a preferred measurement methodand system for locating and measuring X,Y reference locations. Anadditional option to further improve the process energy window ismeasurement and control concept to adjust the laser energy and power asrequired by the material to be processed. If the reflectivity is high,then the energy is to be increased to compensate for these reflectionlosses. If the reflection is low, then the energy and power is to bedecreased since more energy is being coupled into the workpiece ortarget microstructure. There are a number of ways that one can adjustthis power and energy. The simplest is to measure the reflectance fromthe surface and adjust the energy and power control for optimum energycoupling.

[0184] Light interference between metal and oxide layers can greatlyaffect the reflection and hence the absorption in the metal links (seeFIGS. 11 and 12). Even though the process engineer tries to optimize theabsorption in the link by designing the best oxide thickness, thenecessary thickness tolerance is difficult to control. Typically, thethickness of a layer may vary by 10% and there may be several layers ofoxide between the top layer and the metal layer to be processed.

[0185] If the thickness and index refraction over the link could bedetermined, then the energy required to process the link could becalculated and adjusted accordingly. There are two methods ofdetermining the optical constants of a film. These are ellipsometry andspectral analysis. Ellipsometry uses the change in polarization as alight beam either transmits or reflects from a surface. The amount ofchange in polarization determines the index of refraction of thematerial and thickness of the material that the light beam traverses.The spectrometric method measures the reflection from a surface atdifferent wavelengths to determine the same optical constants. Incommercial versions of the spectrometer, the reflected light is sensedat 256 different wavelengths and calculations made on thickness, indexof refraction and extinction coefficient (absorptivity) of the layers tovery high accuracy.

[0186] Another method is to measure the reflectance at two differentwavelengths and calculate the thickness of the oxide. If the index ofrefraction of the oxide used for the device could be measured, then thereflectance and hence the fraction of the laser radiation absorbed overthe link can be calculated. Knowing this absorption, the optimum laserenergy to remove the link can be programmed into the laser system. Thissecond method is more accurate for thin film trimming where the materialto be trimmed is thin and some of the energy is transmitted through thefilm.

[0187] The implementation of the thickness measurement and energycontrol is as shown in FIG. 13. The laser 160 used to remove the linkprovides one of the laser wavelengths for the thickness measurement. Theenergy delivered to the part is controlled by an acousto-opticalmodulator (i.e., “energy control”) 161 as shown in FIG. 13 and isreduced to a level to measure the reflectance without damaging the part.The other wavelength to measure the reflectivity can be provided by ared laser diode (i.e., 670 nm diode) 162 added into the optical path asshown. Beamsplitters 166,167 (e.g., dichroic mirrors) are generally usedto transmit the two wavelengths to the device surface and to direct thereflected beams to the photodiode detectors 164,165. The reflectance canbe monitored by the two photodiodes 164,165 as shown in FIG. 13. Fromthe reflectance intensity of the two photodiodes (i.e., the 670 nm diodeand 1047 nm detectors) and knowing the index of refraction of the oxidelayers, the thickness of the oxide is uniquely determined. Once thethickness and the index of refraction is known, then the absorption inthe link material can be calculated and the optimum energy programmedinto the acousto-optic energy control device by the computer.

[0188] For the highest accuracy, the size of the spot and the linkdimensions can be used in the calculation. Referring to FIGS. 4a and 4b, one sees that there is some energy that will fall off the link andtherefore the difference in the reflected light that does not fall onthe link has to be calculated. Hence, two measurements have to be madeto accommodate for the reflected energy that is not covered by the link.These measurements can be made on each die if required and the energyper pulse can be varied as the thickness of the oxide varies across thewafer. Alternatively, the method could be selectively applied on awafer-by-wafer basis for process monitoring, for instance. Thistechnique will reduce the requirement to use a laser processing energythat is on the high side to account for the variations in absorption inthe link due to interference effects.

[0189] Reflectivity Measurement and Power Adjustment—Case 2: Tunable orAdjustable Wavelength

[0190] The process energy window may be improved in certain cases byadjusting the wavelength over a range wherein the coupling of energy tothe target is improved, the stack reflectance is increased by way of theinterference effect, or where the substrate reflectivity increases.Special solid state tunable lasers—Optical Parametric Oscillators (OPO),Raman, or other tunable lasers may be used provided that power andrepetition rate requirements are met for a given application. Forexample, parametric oscillators may be used which are of fixedwavelengths, that use 2 or 3 crystals at the same time. Under certaincircumstances, tunable lasers are operable. Published U.S. patentapplication 2001-0036206 describes a tunable laser diode having a 40 nmrange developed for the telecommunications industry (i.e. 1.55 μmwavelength). Standard OPO lasers provide for high power and narrowpulses but generally a very slow rep rate, but may be suitable forcertain applications. However, 10 KHz versions have been demonstratedand proposed for 20 KHz repetition rates. U.S. Pat. Nos. 6,334,011 and5,998,759 (Reference 4), and U.S. Pat. No. 6,340,806 (Reference 6)disclose various combinations of shifters. As disclosed in the '759patent, Fosterite lasers have a tunable region that essentiallystraddles the absorption edge region of silicon, and can permitoperation both beyond and below the absorption edge of silicon. At thepresent state of the art, they do not appear to be as efficient as theymay become in time. As materials and improvements are being continuallydeveloped in the laser field, it is within the invention to use suchdevices and obtain corresponding benefits for processing. For instance,the multilayer thickness and reflectivity measurements may be extendedto select a wavelength range which will provide for an improved energywindow.

[0191] Application to a Cu Link with a Single Layer Between theSubstrate and the Link

[0192] It should be noted that the above teachings can also beselectively applied to conventional link structures (see FIG. 2-B), forinstance processing of high reflectivity copper links separated from thesubstrate by a single layer dielectric layer. Production trends arepushing away from polysilicon structures and toward metal structures ofAl and Cu, which poses on-going challenges for link processing systemsto avoid reliability problems and to increase yield. As discussed above,many Cu-based devices have a multi-layer stack wherein substrate andstack damage can be avoided with wavelength selection, spatial beamshaping, or temporal shaping in accordance with the above teachings.However, some manufacturers etch all the dielectrics under the copperlink and build the fuse on a single layer dielectric, with no SiN layersbetween the link and substrate. With conventional laser processing, thelikelihood of substrate damages increases due to the high power requiredfor Cu processing.

[0193] In certain cases processing with multiple pulses (“double blast”)has been used to process metal fuses. However, there is generally athroughput problem for the double blast approach because two passes arerequired in present on-line memory processing systems. Simulated resultsand experiments indicate a second blast may open the link completelyeven if 1st blast failed, despite extended time between first and secondblasts. In specific cases improved yield was reported. According to thesimulation results, double blast with 50% energy of a single blastenergy was very interesting; it was observed that the Si substrate actsas a heat sink and cools down very fast. As shown in FIG. 16, theresults indicated only 10 to 20 ns are needed for the Si substrate 201to stabilize to room temperature. The copper target 202 recovery wasmuch slower indicating a significant differential thermal property. Thesecond pulse will also clear debris at cut site resulting in an “opencircuit”. It is estimated that about 60-70% of the energy used in a“single blast” is needed for each pulse of “double blast.” The pulseenergy may be varied with each pulse. In this example, the pulse delaywas 50 ns, but it is clear that a much shorter delay may be possible.

[0194] In one embodiment, a delay line arrangement of FIG. 15a may beused to avoid any delay in throughput. For example, with a preferredpositioning system of the '118 patent (i.e., Reference 2) assume about150 mm/sec for fine stage speed movement. With 30 ns between two pulses,the change in beam position at the link location would be only 0.0045 umwhich is negligible. In an optical delay line (FIGS. 15b and 15 c, forinstance), 9 meters of extended path in air for the beam will delay 30ns for the second pulse. Alternatively, as shown in FIG. 15a, a secondlaser could be used with a 30 ns or other controllable delay between thetrigger pulses, and the trigger delay may be generated with aprogrammable digital delay line. The temporal pulse shape may be a fastrising, square pulse (as was used in the simulation) generated with aseed laser diode, for instance.

[0195] Numerous options for generating the pulse combinations may beimplemented based on the teachings herein. For example, at least onepulse may have a duration of greater than a few picoseconds to severalnanoseconds. The pulses may be amplified mode locked pulses. At leastone pulse may generated with a q-switched microlaser having a pulsewidthless than 5 nanoseconds. At least one pulse may propagate along a secondoptical path whereby the pulse delay is determined by a difference inoptical path length as shown in FIGS. 15b,c. Multiple laser and/oramplifiers may be used as shown in FIG. 15a.

[0196] As shown in FIG. 18, the generated pulses 275 may have arepetition rate and a corresponding temporal spacing approximately equalto or shorter than a pre-determined delay (e.g., 60 MHz mode lockedsystem) and a modulator is used to select the at least second pulseirradiating the microstructure or groups of pulses 276. U.S. Pat. No.5,998,759 (e.g., Reference 4, col. 7, and the associated drawings)teaches the use of a modulator to allow pulses to irradiate a link ondemand. At very high speed repetition rates an electro-optic modulatoris preferred.

[0197] Additional optics may be used to spatially shape at least one ofthe delayed pulses, prior to combining for instance. For instance, asshown in FIG. 17, a first pulse 210 may be an elliptical or circularGaussian spatial shape, or a top hat along the length of the link. Thesecond pulse 212 may have a different aspect ratio, or may be a specialform of a “cleaning pulse” wherein the central zone of the spot isattenuated with an apodizing filter or effectively removed with acentral obscuration. In such a case, the energy will be concentrated atthe link periphery to remove debris 211 around the link locationresulting from processing with the first pulse, thereby completing theprocessing 213. (For clarity, this “on-the-fly” link site cleaning stepis to be distinguished from the “cleaning for measurement” methoddescribed above). Reference 1 provides at least one example of beamshaping for link blowing applications, wherein a uniform distributionrather than Gaussian spot profile is disclosed.

[0198] In certain cases, the relative motion between the microstructureand the laser beam may be significant between the pulses, e.g., greaterthan 25% of the spot size. This may be the result of a slower repetitionrate (with increased pulse energy), faster motion speed, a longerpre-determined delay, or decreased target area. For example, anultrashort or other short pulse laser system with amplified pulses withoutput energy in the range of several microjoules-millijoules may have a100 KHz-10 MHz repetition rate whereas a system with 10-40 nanojouleoutput may have 50 MHz repetition rate. In the former case, a highspeed, small angle beam deflector may be used to compensate for themotion and deflect a delayed pulse to substantially irradiate the firstmicrostructure at the slower repetition rate during relative motion 258.

[0199] In one embodiment generally illustrated in FIG. 19, the deflectorwould be operatively coupled to the relative positioning systemcontroller 251 in a closed loop arrangement. The deflector is preferablysolid state and may be a single axis acousto-optic device which has avery fast “retrace”/access time. Alternatively, a higher speedelectro-optic deflector (e.g., a gradient index reflector or possibly adigital light deflector) may be used. The time-bandwidth product (numberof spots) can be traded for response time on an application basis. Thedeflector would preferably be used for intensity control and pulsegating/selection, as taught in Reference 4 (col. 7, and associateddrawings). Alternatively, an electro-optic modulator may be used with aseparate acousto-optic deflector operated in a “chirp mode” 252 (e.g.,linear sweep as opposed to random access mode) and synchronized(triggered) 253 based on the positioning system coordinates 254. Thepositioning system coordinates are, in turn, related to the time atwhich the laser pulses are gated by the modulator to irradiate the samesingle microstructure 256 at times t₁, t₂, t₃ corresponding to theselected pulses 259 during relative motion 258.

[0200] In yet another embodiment, a single laser pulse is used to blastup to two links at one time (e.g., no, one or two links). Referring toFIG. 20, two focused spots 306,307 are formed on two links by spatiallysplitting the single collimated laser beam 310 into two divergingcollimated beams 309. The use of acousto-optic devices for spatiallysplitting beams in material processing applications is known in the art.For example, patent abstract JP 53152662 shows one arrangement fordrilling microscopic holes using a multi-frequency deflector havingselectable frequencies f₁ . . . f_(n).

[0201] A laser 300 is pulsed at a predetermined repetition rate. Thelaser beam goes through relay optics 302 that forms an intermediateimage of the laser beam waist into the acoustic optic modulator (AOM)aperture. The AOM 303, which operates in the Bragg regime, preferably isused to controllably generate the two slightly diverging collimatedfirst order diffraction laser beams and control the energy in each beam.The AOM is driven by two frequencies, f₁ and f₂ where f₁=f_(0+Δf) andf₂=f_(0−Δf) where Δf is a small percentage of the original RF signalfrequency f₀. The angle between the two beams is approximately equal tothe Bragg angle for f₀ multiplied by 2(Δf/f₀). The AOM controls theenergy in each of the laser beams by modulating the signal amplitudes oftwo frequency components, f₁ and f₂, in the RF signal and makingadjustments for beam cross-coupling.

[0202] After exiting the AOM, the beams go through the beam rotationcontrol module 313 to rotate the beam 90 degrees on axis with linksorientated in either the X or Y. In one embodiment, a prism is used forthis rotation, though many rotation techniques are well known asdescribed in the regular U.S. application noted in the Cross-Referenceto Related Applications section.

[0203] Next, the beam goes through a set of optics to position the beamwaist and set the beam size to be appropriate for the zoom optics andthe objective lens 305. Note, the zoom optics also modify the anglebetween the two beams, therefore the angle between the two beams exitingthe AOM has to be adjusted depending on the zoom setting to result inthe desired spot separation at the focal plane. Next, the laser beamsenter the objective lens 305 which provides a pair of focused spots 306,307 on two links. The two spots are separated by a distance that isapproximately equal to the focal length of the lens times the anglebetween the two beams. In one exemplary embodiment, a 80 MHz AOM centerfrequency with a sweep range of about 2.3 MHz (77.7-82.3 MHz) may beused to produce a spot size of about 1.8 μm on a pair of adjacent linksspaced apart by about 3 μm. As mentioned earlier, these links may have adimension on the order of a laser wavelength (e.g., 1 micron) which, atvery high speed operation, require the precision positions of the laserbeam and microstructure.

SUMMARY OF SOME GENERAL ASPECTS OF INVENTION

[0204] In summary, one aspect of the invention is a method of selectivematerial processing of a microscopic target structure with a pulsedlaser beam. The target structure is separated from a substrate by aplurality of layers which form a multi-layer stack. The targetstructure, layers, and substrate have dissimilar thermal and opticalproperties. The method includes generating a pulsed laser beam with anenergy density; irradiating the target structure with at least onepulse. Undesirable changes to the stack structure and substrate areavoided by selection of at least one pulse characteristic.

[0205] A portion of the stack may be irradiated with the laser beamduring the processing of the target structure, yet undesirable damage tothe layers, substrate, and functional circuitry in a plane of the innerlayers is avoided.

[0206] Undesirable damage of the stack structure includes cracking,induced by thermal stress, of inner dielectrics. Undesirable damage toinner layer conductors of the stack includes thermal damage caused byirradiation. Undesirable damage to the substrate may arise from laserirradiation and resulting thermal diffusion.

[0207] The dielectric layers may include Silicon Nitride or SiliconDioxide. The substrate may be Silicon.

[0208] The target structure is preferably copper, and may have thicknessor width below one micron, with a dimension at or below wavelengths ofvisible light. Alternatively, the target structure may be a metal link,for instance aluminum, titanium, platinum, or gold.

[0209] An aspect of the invention is selection or control of the spatialand temporal beam characteristics of the pulse, which allows the targetstructure to be cleanly processed while avoiding undesirable damage tothe layers, substrate, and functional circuitry in a plane of the innerlayers.

[0210] A temporal characteristic of the pulse is the pulse shape. Thepulse shape includes a rise time fast enough to efficiently couple laserenergy into the target, a duration sufficient to cleanly remove aportion of the target structure, and a fall time fast enough to avoidundesirable damage caused by subsequent optical transmission. Apreferred pulse rise time for link processing is less than 1 nanosecond(ns) to about 2 ns. A preferred duration is less than 10 ns. A fall timeof less than 3 ns is preferred. The pulse shape may be substantiallysquare, with ringing or variation between the rising and falling edgesof about +−10%. A single pulse or multiple pulses in the form of a rapidburst may be used. Alternatively, a series of q-switched pulses spacedapart in time, with varying output power if desired, may be combined toform a pulse shape having a fast leading edge with high peak power,followed by a second pulse with lower power. In yet another embodimentof the present invention the q-switched pulses may have approximatelythe same output power and combined to produce a substantially squarepulse shape.

[0211] Another temporal pulse characteristic is the pulse power at theleading edge. If the irradiance on the target structure is greater thanabout 10⁹ W/cm², the reflectivity of the target structure is reduced andcoupling of the laser energy is improved.

[0212] A fast rising pulse characteristic avoids undesirable damage of adielectric stack of a memory device having a metal target structure.Cracking of the upper corner occurs during the pulse duration whichlowers the stress on the lower corner adjacent to underlying layers ofthe stack.

[0213] A spatial characteristic of the beam is the irradiance profile ata controlled beam waist position. The irradiance profile may approximatea circular Gaussian beam, an elliptical Gaussian beam, a rectangularprofile in one direction and Gaussian in the orthogonal direction. Thebeam may be nearly diffraction limited. A spatial shape and beamnumerical aperture may be selected to control the interaction of thepulsed laser beam with the target and underlying structures of the 3Ddevice structure to avoid undesirable damage. The material interactionmay further be controlled by precision positioning of the beam waist ofthe pulsed laser beam. The numerical aperture and beam shape may beselected so the spot size and link size are substantially matched in atleast one dimension.

[0214] One aspect of the invention is a method of selection of a pulsecharacteristic based on a model of pulse interaction within a portion ofthe three-dimensional device structure. The three-dimensional deviceincludes a target structure, stack, and substrate with a dissimilaroptical property. A series of structures are disposed at a predeterminedspacing to form an array, with at least one structure not designated asa target structure. A specification may further include informationregarding the material and spacing of functional circuit elements in aplane of the stack. The method includes determining the opticalpropagation characteristics of a portion of an incident pulsed laserbeam which is not absorbed by the target structure. The method furtherincludes specifying a laser pulse characteristic to avoid undesirabledamage to any non-target structures, stack, and substrate.

[0215] The interactions mechanisms which result in selection of a pulsecharacteristic include reflection from the target surface, layer surfaceand internal reflections, polarization, interference effects, near fielddiffraction, scattering and absorption or a combination thereof. Athermal model may be used in conjunction with an optical model.

[0216] The energy in a pulse used for processing a copper link targetstructure of a semiconductor memory device may be in the range of about0.1-5 microjoules. The energy density corresponds to an area of theirradiance profile of the beam waist. The area may be in the range ofless than 20 square microns, and preferably less than 10 square microns.

[0217] Another controllable laser pulse characteristic is polarization.The polarization may be controlled or selected based on the relativereflectance of the layers and optical coupling of laser energy into thetarget structure at a wavelength.

[0218] A wavelength of the laser pulse may be selected based on thereflectance of the multi-layer stack (interference effect). Thepreferred wavelength corresponds to a spectral region where the stackreflection is substantial, for example 60%, and where the internaltransmission of within a layer of the stack is high, approaching amaximum. Short wavelengths are preferred for maximum control of thespatial characteristics of the beam (for example, the smallestachievable beam waist with an option for controllably selecting a largerbeam waist and depth of focus). The laser wavelength may be fixed, ormay be varied with wavelength shifting or harmonic generation. Ameasurement of the thickness or reflectance may be used to select oradjust the wavelength.

[0219] In at least one embodiment, the target structure may besubstantially reflective at the laser wavelength. The laser wavelengthmay be below the absorption edge of the substrate and correspond to anabsorbing or reflecting region. The laser wavelength is above theabsorption edge of the dielectrics layers of the stack, and correspondsto a substantially maximum transmitting region.

[0220] A selected wavelength corresponds to the near UV, visible andnear IR spectrum, from below 0.4 μm to about 1.55 μm. The lower limitmay be determined by the absorption of a layer. With silicon substrates,both absorption and reflection increase at shorter wavelengths. ForSilicon Dioxide and Silicon Nitride, the internal transmission andsingle surface reflectance are substantially constant throughout thevisible and near IR ranges. The upper limit corresponds to a range ofpreferred laser wavelengths of laser diodes, optical amplifiers. Anamplifier output may be either wavelength preserved or Raman shifted.

[0221] Another aspect of the invention is a method of selective materialprocessing of a microscopic target structure of a multi-material,multi-layer device with a pulsed laser beam. The target structure,layers, and substrate have a dissimilar thermal and optical property.The beam has a focused beam waist with a centerline. An alignmentpattern is included at one of a plurality of predetermined measurementlocations associated with the device. The alignment pattern is coveredby at least one layer. The target structure is separated from asubstrate by a plurality of layers which form a multi-layer stack. Themethod includes measuring the position of the alignment target in atleast one dimension; predicting the relative location of the targetstructure and centerline based on the measurement; inducing relativemotion between the target structure and the centerline based on themeasurement; generating a pulsed laser beam with an energy density;irradiating the target structure with at least one pulse. Undesirablechanges to the stack structure and substrate are avoided with byselection of a pulse characteristic.

[0222] The measurement of a position may include a method and system forpolarization insensitive detection to avoid spurious measurementsresulting from reflected signal variations. The signal variations mayresult from process induced optical characteristics, includingbirefringence.

[0223] The relative location of the target structure, beam waist, andcenterline may be predicted based on multi-parameter least squares fit.

[0224] A cleaning process may be used to enhance data used formeasurement by removing contaminants which produce multiplicativevariations (reflection noise).

[0225] Three-dimensional (depth) measurements may be done using thealignment target, wafer, or other suitable material. The measurement maybe used to predict the relative location of the target structurerelative to the beam waist, the beam waist being located along thecenterline of the pulsed laser beam. A surface may be estimated from thethree-dimensional measurements. A numerical offset may be introduced tocompensate for a depth difference between a measurement location and thetarget structure, based on the thickness of the stack.

[0226] An aspect of the invention includes measurement of the layerthickness or reflectivity at a location, and use of the measurement tocontrol a pulse characteristic. The pulse characteristic may be thepulse energy, pulse width, or wavelength. The location may be a singlelocation on the device or a plurality of locations.

[0227] While the best modes for carrying out the invention have beendescribed in detail, those familiar with the art to which this inventionrelates will recognize various alternative designs and embodiments forpracticing the invention as defined by the following claims.

What is claimed is:
 1. A method for thermal-based laser processing amulti-material device including a substrate and at least onemicrostructure, the processing occurring with multiple pulses in asingle pass operation controlled with a positioning subsystem of athermal processing system, the positioning subsystem inducing relativemotion between the device and laser beam waists, the processing toremove the at least one microstructure without damaging the substrate,the method comprising: generating a first pulse having a firstpredetermined characteristic; irradiating the at least onemicrostructure with the first pulse wherein a first beam waistassociated with the first pulse and the at least one microstructuresubstantially coincide, the step of irradiating at least initiatingprocessing the at least one microstructure; generating a second pulsehaving a second predetermined characteristic, the second pulse beingdelayed a predetermined time relative to the first pulse; andirradiating the at least one microstructure with the second pulsewherein a second beam waist associated with the second pulse and the atleast one microstructure substantially coincide, the step of irradiatingthe at least one microstructure with the second pulse further processingthe at least one microstructure wherein the processing of the at leastone microstructure with the first and second pulses occurs duringrelative motion of the at least one microstructure and the beam waistsin a single pass whereby throughput of the thermal processing system issubstantially improved.
 2. The method of claim 1 wherein the device is asemiconductor memory including a silicon substrate and wherein the atleast one microstructure is a metal link of the semiconductor memoryseparated from the silicon substrate by at least one oxide layer.
 3. Themethod of claim 1 wherein at least one of the pulses has a duration ofgreater than a few picoseconds to several nanoseconds.
 4. The method ofclaim 1 wherein the pulses are generated by a mode-locked laser systemand amplified with an optical amplifier.
 5. The method of claim 1wherein at least one of the pulses is generated by a q-switchedmicrolaser having a pulsewidth less than 5 nanoseconds.
 6. The method ofclaim 1 wherein the first and second pulses are propagated alongdifferent optical paths so that the second pulse is delayed for thepredetermined time relative to the first pulse based on a difference inoptical path length.
 7. The method of claim 1 wherein the pulses have atemporal spacing less than or approximately equal to the predeterminedtime, and wherein the method further comprises selecting the secondpulse to irradiate the at least one microstructure.
 8. The method ofclaim 1 wherein the predetermined time is determined by a thermalproperty of the substrate wherein substrate temperature is substantiallyreduced after the predetermined time compared to the temperature of thesubstrate during the step of irradiating the at least one microstructurewith the second pulse.
 9. The method of claim 8 wherein the substratetemperature is substantially reduced to approximately room temperature.10. The method of claim 1 wherein the first and second predeterminedcharacteristics comprise a substantially square temporal pulse shapehaving a rise time of less than about 2 nanoseconds and a pulse durationof about 10 nanoseconds.
 11. The method of claim 1 wherein thepredetermined time is in the range of about 20-50 nanoseconds.
 12. Themethod of claim 1 wherein the predetermined time is about 30nanoseconds.
 13. The method of claim 1 wherein two pulses are used tocompletely process the at least one microstructure, and wherein laserenergy of each of the pulses is about 60-70% of laser energy requiredfor laser processing the at least one microstructure with a singlepulse.
 14. The method of claim 1 wherein relative position changebetween the pulses at the at least one microstructure is less than about10% of a dimension of the at least one microstructure to be processed.15. The method of claim 1 wherein at least one of the first and secondpredetermined characteristics comprises a substantially square pulse.16. The method of claim 1 wherein at least one of the predeterminedcharacteristics comprises a non-circular spatial profile based on aselected numerical aperture and shape of a spot and wherein the spot andthe at least one microstructure are substantially correlated in at leastone dimension whereby percent of laser energy delivered to the at leastone microstructure is increased and irradiance of the substrate isdecreased.
 17. The method of claim 1 wherein a spatial beam shape of thesecond pulse is in the form of a cleaning beam having an energy densitylower than energy density of the first pulse.
 18. The method of claim 17wherein the cleaning beam has an attenuated central region and a higherenergy outer region so as to remove debris surrounding a target site onthe at least one microstructure.
 19. The method of claim 1 wherein thesteps of generating include directing a portion of a laser pulse throughan optical subsystem having opposing, spaced-apart, corner cubereflectors and polarization rotators so as to align a pulsed laser beam,and to control delay and amplitude of the second pulse relative to thefirst pulse.
 20. The method of claim 1 wherein the steps of generatinginclude providing an optical subsystem having multiple lasers whereindelay between trigger pulses to the optical subsystem determines thepredetermined time.
 21. The method of claim 1 wherein a fiber opticdelay line delays the second pulse for the predetermined time andwherein the predetermined time is about several nanoseconds.
 22. Themethod of claim 1 wherein relative position change between the pulses atthe at least one microstructure is either greater than about 10% of adimension of the at least one microstructure to be processed or greaterthan about ½ of either of the beam waists and further including a highspeed beam deflector operatively coupled to the positioning subsystem tocompensate for relative motion between the pulses, wherein the secondpulse is deflected by the deflector to also substantially irradiate theat least one microstructure with the second pulse.
 23. The method ofclaim 1 wherein the predetermined time is in the range of about 10 ns to10 μs.
 24. The method of claim 22 wherein the beam deflector is a singleaxis acousto-optic device.
 25. The method of claim 1 wherein the firstand second predetermined characteristics are based on physicalproperties of the multi-material device.
 26. The method of claim 1wherein the first pulse irradiates a first portion of the at least onemicrostructure and the second pulse irradiates a second portion of theat least one microstructure and wherein relative position change betweenthe first and second portions of the at least one microstructure is lessthan ¼ of either of the beam waists.
 27. The method of claim 20 whereinthe step of providing also provides at least one optical amplifieroptically coupled to at least one of the lasers.
 28. The method of claim1 wherein the at least one microstructure and the beam waists arerelatively positioned during relative motion based uponthree-dimensional information.
 29. The method of claim 1 wherein thesteps of generating includes generating a single pulse and forming thefirst and second pulses from the single pulse.
 30. The method of claim29 wherein the step of forming delays the second pulse for thepredetermined time relative to the first pulse.
 31. The method of claim29 wherein the step of forming includes splitting the single pulse witha multi-frequency deflector to form the first and second pulses.
 32. Themethod of claim 31 wherein first and second microstructures areirradiated by the first and second pulses, respectively.
 33. A systemfor thermal-based laser processing a multi-material device including asubstrate and at least one microstructure, the processing occurring withmultiple pulses in a single pass operation controlled with a positioningsubsystem which induces relative motion between the device and laserbeam waists, the processing to remove the at least one microstructurewithout damaging the substrate, the system comprising: means forgenerating a first pulse having a first predetermined characteristic;means for irradiating the at least one microstructure with the firstpulse wherein a first beam waist associated with the first pulse and theat least one microstructure substantially coincide, the first pulse atleast initiating processing the at least one microstructure; means forgenerating a second pulse having a second predetermined characteristic,the second pulse being delayed a predetermined time relative to thefirst pulse; and means for irradiating the at least one microstructurewith the second pulse wherein a second beam waist associated with thesecond pulse and the at least one microstructure substantially coincide,the second pulse further processing the at least one microstructurewherein the processing of the at least one microstructure with the firstand second pulses occurs during relative motion of the at least onemicrostructure and the beam waists in a single pass whereby throughputof the system is substantially improved.
 34. The system of claim 33wherein the device is a semiconductor memory including a siliconsubstrate and wherein the at least one microstructure is a metal link ofthe semiconductor memory separated from the silicon substrate by atleast one oxide layer.
 35. The system of claim 33 wherein at least oneof the pulses has a duration of greater than a few picoseconds toseveral nanoseconds.
 36. The system of claim 33 wherein the means forgenerating includes a mode-locked laser system and further comprising anoptical amplifier for amplifying the pulses.
 37. The system of claim 33wherein at least one of the means for generating includes a q-switchedmicrolaser having a pulsewidth less than 5 nanoseconds.
 38. The systemof claim 33 wherein the first and second pulses are propagated alongdifferent optical paths so that the second pulse is delayed for thepredetermined time relative to the first pulse based on a difference inoptical path length.
 39. The system of claim 33 wherein the pulses havea temporal spacing less than or approximately equal to the predeterminedtime, and wherein the system further comprises means for selecting thesecond pulse to irradiate the at least one microstructure.
 40. Thesystem of claim 33 wherein the predetermined time is determined by athermal property of the substrate wherein substrate temperature issubstantially reduced after the predetermined time compared to thetemperature of the substrate during irradiation of the at least onemicrostructure with the second pulse.
 41. The system of claim 40 whereinthe substrate temperature is substantially reduced to approximately roomtemperature.
 42. The system of claim 33 wherein the first and secondpredetermined characteristics comprise a substantially square temporalpulse shape having a rise time of less than about 2 nanoseconds and apulse duration of about 10 nanoseconds.
 43. The system-of claim 33wherein the predetermined time is in the range of about 20-50nanoseconds.
 44. The system of claim 33 wherein the predetermined timeis about 30 nanoseconds.
 45. The system of claim 33 wherein two pulsesare used to completely process the at least one microstructure, andwherein laser energy of each of the pulses is about 60-70% of laserenergy required for laser processing the at least one microstructurewith a single pulse.
 46. The system of claim 33 wherein relativeposition change between the pulses at the at least one microstructure isless than about 10% of a dimension of the at least one microstructure tobe processed.
 47. The system of claim 33 wherein at least one of thefirst and second predetermined characteristics comprises a substantiallysquare pulse.
 48. The system of claim 33 wherein at least one of thepredetermined characteristics comprises a non-circular spatial profilebased on a selected numerical aperture and shape of a spot and whereinthe spot and the at least one microstructure are substantiallycorrelated in at least one dimension whereby percent of laser energydelivered to the at least one microstructure is increased and irradianceof the substrate is decreased.
 49. The system of claim 48 wherein aspatial beam shape of the second pulse is in the form of a cleaning beamhaving an energy density lower than energy density of the first pulse.50. The system of claim 49 wherein the cleaning beam has an attenuatedcentral region and a higher energy outer region so as to remove debrissurrounding a target site on the at least one microstructure.
 51. Thesystem of claim 33 wherein the means for generating the first and secondpulses include an optical subsystem having opposing, spaced-apart,corner cube reflectors and polarization rotators so as to align a pulsedlaser beam, and to control delay and amplitude of the second pulserelative to the first pulse.
 52. The system of claim 33 wherein themeans for generating the first and second pulses include an opticalsubsystem having multiple lasers wherein delay between trigger pulses tothe optical subsystem determines the predetermined time.
 53. The systemof claim 33 further comprising a fiber optic delay line to delay thesecond pulse for the predetermined time and wherein the predeterminedtime is about several nanoseconds.
 54. The system of claim 33 whereinrelative position change between the pulses at the at least onemicrostructure is either greater than about 10% of a dimension of the atleast one microstructure to be processed or greater than about ½ ofeither of the beam waists and further including a high speed beamdeflector operatively coupled to the positioning subsystem to compensatefor relative motion between the pulses, wherein the second pulse isdeflected by the deflector to also substantially irradiate the at leastone microstructure with the second pulse.
 55. The system of claim 33wherein the predetermined time is in the range of about 10 ns to 10 μs.56. The system of claim 54 wherein the beam deflector is a single axisacousto-optic device.
 57. The system of claim 33 wherein the first andsecond predetermined characteristics are based on physical properties ofthe multi-material device.
 58. The system of claim 33 wherein the firstpulse irradiates a first portion of the at least one microstructure andthe second pulse irradiates a second portion of the at least onemicrostructure and wherein relative position change between the firstand second portions of the at least one microstructure is less than ¼ ofeither of the beam waists.
 59. The system of claim 52 further comprisingat least one optical amplifier optically coupled to at least one of thelasers.
 60. The system of claim 33 wherein the at least onemicrostructure and the beam waists are relatively positioned duringrelative motion based upon three-dimensional information.
 61. The systemof claim 33 wherein the means for generating the first and second pulsesincludes means for generating a single pulse and means for forming thefirst and second pulses from the single pulse.
 62. The system of claim61 wherein the means for forming delays the second pulse for thepredetermined time relative to the first pulse.
 63. The system of claim61 wherein the means for forming includes a multi-frequency deflectorfor splitting the single pulse to form the first and second pulses. 64.The system of claim 63 wherein first and second microstructures areirradiated by the first and second pulses, respectively.
 65. A methodfor thermal-based laser processing a multi-material device including asubstrate and a microstructure, the method comprising: generating the atleast one laser pulse having at least one predetermined characteristicbased on a differential thermal property of materials of the device; andirradiating the microstructure with the at least one laser pulse whereina first portion of the at least one pulse increases a difference intemperature between the substrate and the microstructure and wherein asecond portion of the at least one pulse further increases thedifference in temperature between the substrate and the microstructureto process the multi-material device without damaging the substrate. 66.The method as claimed in claim 65 wherein the first and second portionsare portions of a single pulse.
 67. The method as claimed in claim 65wherein the first and second portions are portions of different pulses.68. The method as claimed in claim 65 wherein the first portion of theat least one pulse increases temperature of the microstructure.
 69. Themethod as claimed in claim 65 wherein the first portion is a highdensity leading edge portion of the at least one pulse.
 70. The methodas claimed in claim 69 wherein the leading edge portion has a rise timeof less than two nanoseconds.
 71. The method as claimed in claim 70wherein the rise time is less than one nanosecond.
 72. The method asclaimed in claim 65 wherein the first and second portions of the atleast one pulse are sufficient to remove the microstructure.
 73. Themethod as claimed in claim 69 wherein the microstructure is a metal linkhaving reflectivity and wherein the leading edge portion of the at leastone pulse reduces the reflectivity of the metal link.
 74. The method asclaimed in claim 73 wherein the substrate is silicon and the device is asemiconductor memory.
 75. The method as claimed in claim 68 wherein thesecond portion of the at least one pulse further increases thetemperature of the microstructure.
 76. The method as claimed in claim 65wherein the step of irradiating is completed in a period between 5 and75 nanoseconds.
 77. The method as claimed in claim 76 wherein the periodis between 10 and 50 nanoseconds.
 78. A system for thermal-based laserprocessing a multi-material device including a substrate and amicrostructure, the system comprising: means for generating the at leastone laser pulse having at least one predetermined characteristic basedon a differential thermal property of materials of the device; and meansfor irradiating the microstructure with the at least one laser pulsewherein a first portion of the at least one pulse increases a differencein temperature between the substrate and the microstructure and whereina second portion of the at least one pulse further increases thedifference in temperature between the substrate and the microstructureto process the multi-material device without damaging the substrate. 79.The system as claimed in claim 78 wherein the first and second portionsare portions of a single pulse.
 80. The system as claimed in claim 78wherein the first and second portions are portions of different pulses.81. The system as claimed in claim 78 wherein the first portion of theat least one pulse increases temperature of the microstructure.
 82. Thesystem as claimed in claim 78 wherein the first portion is a highdensity leading edge portion of the at least one pulse.
 83. The systemas claimed in claim 82 wherein the leading edge portion has a rise timeof less than two nanoseconds.
 84. The system as claimed in claim 83wherein the rise time is less than one nanosecond.
 85. The system asclaimed in claim 78 wherein the first and second portions of the atleast one pulse are sufficient to remove the microstructure.
 86. Thesystem as claimed in claim 82 wherein the microstructure is a metal linkhaving reflectivity and wherein the leading edge portion of the at leastone pulse reduces the reflectivity of the metal link.
 87. The system asclaimed in claim 86 wherein the substrate is silicon and the device is asemiconductor memory.
 88. The system as claimed in claim 81 wherein thesecond portion of the at least one pulse further increases thetemperature of the microstructure.
 89. The system as claimed in claim 78wherein the irradiation is completed in a period between 5 and 75nanoseconds.
 90. The system as claimed in claim 89 wherein the period isbetween 10 and 50 nanoseconds.